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D M Bers

Publications and source records attributed to D M Bers.

144 records · Page 8Linked to original sources
PubMed1987

Inotropic response to hypothermia and the temperature-dependence of ryanodine action in isolated rabbit and rat ventricular muscle: implications for excitation-contraction coupling.

We have used the sarcoplasmic reticulum (SR) inhibitor ryanodine to assess the contribution of the SR to the increase in twitch tension seen on cooling the mammalian myocardium. To select a suitable concentration of ryanodine, i.e., one that will exert a maximal effect at all temperatures studied, concentration-response curves for ryanodine action were constructed at 37 degrees, 29 degrees, and 23 degrees C in ventricular muscle from rabbit and rat. Using a concentration of ryanodine (1 microM) that exerted a maximal effect at all temperatures studied, the ability of ryanodine to inhibit SR function at 37 degrees, 29 degrees, and 23 degrees C was then confirmed by using rapid cooling contractures (RCCs) to provide an indirect assessment of the SR calcium content. To estimate the rest decay of the SR calcium content in the absence and presence of ryanodine (1 microM), RCCs were initiated after a range of rest intervals (0.3-300 seconds) in rabbit muscles maintained at 37 degrees, 29 degrees, or 23 degrees C. In the absence of ryanodine, low temperatures elevated RCCs at all rest intervals studied. In the presence of ryanodine, RCCs were only seen at rest intervals shorter than 2.0 seconds, even at 23 degrees C, the lowest temperature studied. Thus, even at 23 degrees C, ryanodine appears to be effective at inhibiting SR calcium release in muscles stimulated at 0.5 Hz (i.e., after 2 seconds rest). Therefore, using this concentration of ryanodine (1 microM) and a stimulation rate of 0.5 Hz, we have investigated the contribution of the SR to the positive inotropic response to hypothermia. Under these conditions, the positive inotropic response to cooling in rabbit ventricle was almost unaffected by the inhibition of the SR with ryanodine. In rat ventricle, a tissue in which SR calcium release may dominate excitation-contraction (EC) coupling, the inotropic response to hypothermia was still observed, although developed tension was strongly depressed at all temperatures. These results suggest that a change in SR function is not the principal mediator of the large (400-500%) increase in force associated with cooling mammalian ventricular muscle from 37 degrees to 25 degrees C. The ryanodine-sensitive fraction of tension development was greatest at 37 degrees C, suggesting that the relative contribution of the SR to tension development in rabbit ventricle is reduced at temperatures below 37 degrees C.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1986

Calcium binding to cardiac sarcolemmal vesicles: potential role as a modifier of contraction.

Passive Ca binding to cardiac sarcolemmal vesicles isolated from rabbit ventricles was measured under ionic conditions similar to intracellular and extracellular media. The first of two main goals was to evaluate whether certain agents induce changes in Ca binding at the external sarcolemmal surface that might contribute to the overall effect of these agents on cardiac muscle contraction. The agents studied were ouabain, verapamil, nifedipine, Bay K 8644, caffeine, ryanodine, and milrinone over a broad range of concentrations, including concentrations at which these agents exert strong effects on cardiac contractile performance. None of these agents produced significant alterations in Ca binding, such that it is unlikely that any part of their actions can be attributed to changes in Ca binding to the external sarcolemmal surface. In contrast, when [Na] is reduced from 140 mM, sarcolemmal Ca binding increases or decreases depending on what replacement is used to avoid changes of osmolarity. Thus the possible effect of Na reduction on surface Ca must be considered in physiological experiments where extracellular [Na] is changed. The second main goal was to evaluate the effects of membrane potential, Na and Mg on Ca bound to the inner surface of the sarcolemma under ionic conditions similar to those expected intracellularly (e.g., [Ca] = 0.3-5.0 microM). Ca binding was inhibited by physiological concentrations of Na and Mg and was sensitive to membrane potential such that depolarization of a normally polarized cell would cause Ca to be released from these sarcolemmal sites. From a quantitative standpoint, it is not clear whether the effect of depolarization would be to contribute sarcolemmal Ca to the activation of the myofilaments or merely to limit the ability of the inner sarcolemmal surface to buffer the rise in intracellular [Ca] associated with contraction.

Alamethicin↗
PubMed1986

Postrest inotropy in rabbit ventricle: Na+-Ca2+ exchange determines sarcoplasmic reticulum Ca2+ content.

To determine whether Na+-Ca2+ exchange is a physiologically significant Ca2+ efflux mechanism in rabbit ventricle, we investigated the effects exerted on postrest contractions by interventions that alter the transmembrane distribution of Na+ or Ca2+ so as to retard Ca2+ efflux via this system. Contractions elicited after rest periods of 0.25-10 min in duration were studied. The following interventions increased postrest contractions much more than those elicited by rhythmic stimulation: 1) Na+ pump inhibition by cardiac glycosides or by a reduction in extracellular K+, 2) reduction of extracellular Na+ (maintaining a constant [Ca2+]-to-[Na+]2 ratio), and 3) elevation of extracellular Ca2+. In contrast, isoproterenol, norepinephrine, and histamine produced comparable increases in both rhythmically stimulated and postrest contractions, suggesting that the postrest contractile potentiation was not just the result of a general increase in inotropic state. Ryanodine, which appears to antagonize sarcoplasmic reticulum (SR) Ca2+ release in cardiac muscle, markedly reduced the amplitude of the postrest contractions, but only modestly decreased rhythmically stimulated responses. Results suggest 1) that Ca2+ released from SR is involved in postrest response, 2) that Na+-Ca2+ exchange serves as a Ca2+ efflux pathway in normally polarized resting rabbit ventricle, and 3) that this activity in part determines the amount of Ca2+ available for release from SR.

Animals↗
PubMed1986

Cumulative depletions of extracellular calcium in rabbit ventricular muscle monitored with calcium-selective microelectrodes.

Transient changes of extracellular free calcium in rabbit ventricular muscle under nonsteady state conditions were measured with double-barreled calcium microelectrodes. Resumption of stimulation after a rest interval produces a cumulative decrease of extracellular free calcium often by more than 10% (with bulk extracellular free calcium = 0.2 mM). The extracellular free calcium returns to the bulk value as a new steady state is achieved. The changes of extracellular free calcium recorded presumably represent net calcium uptake and loss by cardiac muscle cells. These cumulative extracellular free calcium depletions are blocked by 0.5 mM cobalt and 1 microM nifedipine and are increased to 167 +/- 11% of control by the calcium agonist Bay k 8644 (1 microM) and to 620 +/- 150% of control by increasing stimulus frequency from 0.2-2 Hz. Caffeine (10 mM) inhibits the cumulative extracellular free calcium depletions, probably by rendering the sarcoplasmic reticulum unable to accumulate calcium. It is proposed that the extracellular free calcium depletions recorded represent, in large part, calcium which has entered the cells and has been taken up by the sarcoplasmic reticulum (which had become depleted of calcium during the rest interval). Nifedipine and cobalt inhibit these cumulative depletions presumably by preventing the calcium entry which could subsequently be accumulated by the sarcoplasmic reticulum. The net cellular calcium uptake produced by such a post-rest stimulation protocol can also be inhibited by 1-3 microM acetylstrophanthidin and reduction of extracellular sodium to 70 mM. Acetylstrophanthidin and low extracellular sodium would be expected to shift the sodium-calcium exchange in favor of increased calcium uptake, which may, in turn, prevent the loss of sarcoplasmic reticulum calcium during the rest interval. This would limit the amount of calcium which the sarcoplasmic reticulum could take up with subsequent activation. In contrast to the results with caffeine, ryanodine (1 microM) increases the magnitude and rate of calcium uptake after a rest interval, indicative of a fundamental difference in the actions of caffeine and ryanodine. When stimulation is stopped in the presence of ryanodine, extracellular free calcium increases much faster than in control. This suggests that ryanodine may enhance calcium uptake by the sarcoplasmic reticulum during repetitive stimulation and may enhance calcium efflux from the sarcoplasmic reticulum during quiescence. These experiments provide insight into transsarcolemmal calcium movements and certain aspects of cellular calcium regulation.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1986

Biphasic contractions induced by milrinone at low temperature in ferret ventricular muscle: role of the sarcoplasmic reticulum and transmembrane calcium influx.

The effects of milrinone were studied in ferret papillary muscle stimulated at various rates and temperatures from 23 degrees to 36 degrees C. In voltage-clamp experiments, 50 micrograms/ml (0.237 mM) milrinone induced a 2.1-fold increase in calcium current at 28 degrees or 36 degrees C. At 50 micrograms/ml, milrinone transiently increased contractility in all muscles at 28 degrees C, but its steady-state effect was either increased (+50%) or decreased (-24.7%) steady-state twitch amplitude. A negative inotropic effect always occurred below 27 degrees C. Milrinone decreased the total twitch duration and split the twitch into two components (P1 and P2) in the absence of any evidence of aberrant conduction. Increasing milrinone concentration from 50 to 300 micrograms/ml decreased P1 and increased P2. Ryanodine (100 mM) or caffeine (10 mM) suppressed P1. Contractions elicited after 30 seconds of rest were also biphasic in the presence of milrinone, but not in its absence. P2 of post-rest contraction was increased by caffeine or calcium (10 mM) and decreased by cobalt (2 mM) when drugs were applied at the beginning of the rest. Ryanodine and caffeine also suppressed P1 of post-rest contraction. The evidence suggests that P1 may be caused by Ca release from the sarcoplasmic reticulum and P2 by increased Ca influx during the action potential via the calcium channel. It is also suggested that P2 may be present under control conditions, but to a lesser extent, and masked by a large P1.

Action Potentials↗
PubMed1985

Calcium at the surface of cardiac plasma membrane vesicles: cation binding, surface charge screening, and Na-Ca exchange.

Calcium binding and Na-Ca exchange activity were measured in isolated cardiac plasma membrane vesicles under various ionic conditions. A model was developed to describe the Ca binding characteristics of cardiac sarcolemmal vesicles using the Gouy-Chapman theory of the diffuse double layer with specific cation binding to phospholipid carboxyl and phosphate groups. The surface association constants used for Ca, Na, K and H binding to both of these groups were 7,0.63, 0.3 and 3800 M-1, respectively. This model allows the estimation of surface [Ca] under any specific ionic conditions. The effects of the divalent screening cation, dimethonium, on Ca binding and Na-Ca exchange were compared. Dimethonium had no significant effect on Ca binding at high ionic strength (150 mM KCl), but strongly depressed Ca binding at low ionic strength. Dimethonium had no significant effect on Na-Ca exchange (Na-inside dependent Ca influx) at either high or low ionic strength. These results suggest that the Ca sites of the Na-Ca exchanger are in a physical environment where they are either not exposed to or not sensitive to surface [Ca].

Animals↗
PubMed1985

Ca influx and sarcoplasmic reticulum Ca release in cardiac muscle activation during postrest recovery.

The recovery of twitch tension (T) and Ca influx after 5-s to 10-min rest intervals was studied in rabbit, rat, and frog cardiac muscle. Extracellular double-barreled Ca-selective microelectrodes were used to assess transsarcolemmal Ca influx. In rabbit ventricle, T at the first postrest beat (B1) exceeded T at the second beat (B2). T and Ca influx then increase toward steady state in a parallel fashion. Since Ca influx is small at B1, this beat may be more dependent on Ca stored in the sarcoplasmic reticulum (SR), and subsequent beats may more closely reflect Ca influx. We have examined this hypothesis using agents that may preferentially inhibit either SR Ca release or Ca influx. SR inhibitors caffeine (5 mM) and ryanodine (100 nM) changed the pattern of T recovery so that B1 was now less than B2 and more closely paralleled the recovery of Ca influx. Caffeine and ryanodine decreased steady-state T to 89 +/- 9 and 78 +/- 3%, respectively. In contrast, B1 was found to be less susceptible to inhibition than B2 when exposed to La or Co (influx inhibitors). These results are consistent with a rabbit ventricle model in which the SR Ca release contributes greatly to B1, the SR contribution declines while the Ca influx component increases, and Ca influx at steady state can account for a large fraction of steady-state T. In adult rat ventricle, caffeine and ryanodine decrease steady-state T to 33 +/- 2 and 13 +/- 3%, respectively, changing the T recovery pattern from a monotonic decrease to a monotonic increase. These results are consistent with a model qualitatively the same as rabbit ventricle. However, the SR can account for most of steady-state T in the rat, and thus its decaying characteristic dominates the control postrest recovery. By inhibition of SR Ca release, the increasing component ascribed to Ca influx becomes apparent. Similar experiments were done to establish an order of relative dependence on SR Ca release: adult rat ventricle greater than rabbit atrium greater than rabbit ventricle greater than frog ventricle.

Animals↗
PubMed1985

The role of Ca influx in cardiac muscle excitation-contraction coupling. Assessment by extracellular Ca microelectrodes.

Tension, dT/dt, and extracellular free [Ca] [( Ca]0) were continuously measured in isolated rabbit right papillary muscles at 28 degrees C. Double-barreled Ca-selective microelectrodes incorporating Simon's neutral Ca exchange resin (ETH-1001) were used to monitor [Ca]0. Decreases of [Ca]0 were seen during the course of single twitches, before the development of significant tension. This depletion of Ca0 probably represents Ca influx into the cells from the extracellular space. The magnitude of this Ca0 depletion is decreased by Co, verapamil, long rest intervals, and reduction of [Ca]0. The magnitude is increased by catecholamines, reduction of [Na]0, caffeine, continued pacing, and elevation of [Ca]0. After 3-min rest, stimulation (0.5-1 Hz) produces a biphasic tension response (larger first beat, small second beat, and monotonic rise to control). Caffeine (5 mM) changes the pattern after rest to a monotonic increase. Ca influx increases monotonically in both cases. Addition of 20 mM Co during the rest reduces tension of all beats by similar amounts in the presence of caffeine. In the absence of caffeine, Co has a much weaker effect on the first beat than on subsequent beats. The results suggest that caffeine inhibits an intracellular component of activator Ca that is more important after a rest interval, but that Ca influx becomes increasingly more important during continued pacing under the conditions used here.

Animals↗
PubMed1983

Early transient depletion of extracellular Ca during individual cardiac muscle contractions.

Extracellular free [Ca] in rabbit papillary muscles was monitored using double-barreled Ca microelectrodes. These electrodes had tip diameters of 4-12 microns, electrical time constants of 2-5 ms, and electrochemical time constants of less than 30 ms. During individual beats a transient depletion of extracellular Ca (CaO) was recorded. This decrease of [Ca]O begins very early during the action potential, before significant tension development, and reaches a maximum much before the peak of developed tension (T). The depletion of CaO is blocked by CoCl2 or verapamil and enhanced by 10(-8) M isoproterenol or reduction of extracellular Na concentration to 35 mM. The magnitude of this early depletion of CaO increases in parallel with tension as a function of [Ca]O (8.46 +/- 0.98 microM at 0.2 mM CaO, 16.9 +/- 1.6 microM at 0.5 mM CaO, and 44.7 +/- 3.7 microM at 2.0 mM CaO). However the magnitude (in mV) of the recorded signal decreases with increasing [Ca]O and T, suggestive of saturation. The magnitude of this early transient CaO depletion also increases in parallel with the increase of T produced by initiating stimulation from rest (except for the first beat, which may be more dependent on stored Ca). It is probable that the depletions recorded represent Ca influx into cardiac cells from the extracellular space. The magnitude of Ca influx represented by the CaO depletions is difficult to quantitate but may be roughly in the range of Ca entry that would be required for direct activation of the myofilaments.

Action Potentials↗
PubMed1982

Intracellular calcium and sodium activity in sheep heart Purkinje fibres. Effect of changes of external sodium and intracellular pH.

Intracellular Ca, Na and H selective microelectrodes were used to study the effects of reduction of the extracellular Na concentration, [Na]0, on the free intracellular Ca concentration, [Ca]i, Na activity, (aiNa), and intracellular pH (pHi) in sheep heart Purkinje fibres. 1. Reduction of [Na]0 from 140 mM to 14 mM produced a decrease of aiNa, and increase of free [Ca]i, and normally an increase of resting tension. 2. Inhibition of the Na-K pump by 10(-5) M acetyl-strophanthidin produced a slow rise of [Ca]i and resting tension. 3. The magnitude of the increase of free [Ca]i (and tension) produced by [Na]0 reduction was greatly enhanced when the Na-K pump is inhibited by either acetylstrophanthidin or K-free solutions. 4. We suggest that this enhanced rise of free [Ca]i in the presence of Na-K pump inhibition is due to Ca loading of intracellular Ca buffering systems during the pump inhibition. 5. Addition of NH4Cl produced a transient decrease of free [Ca]i that accompanied an alkaline change in pHi. Removal of NH4Cl (which produced a transient intracellular acidification) produced a transient increase of free [Ca]i. We conclude that a close relationship exists between the control of free [Ca]i and pHi which may be due to competition at, or common use of, intracellular buffering systems.

Animals↗
PubMed1982

A simple method for the accurate determination of free [Ca] in Ca-EGTA solutions.

A simple method for the accurate determination of free [Ca] in ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA)-buffered Ca solutions is described. This method is useful for calibration of Ca macro- and microelectrodes to low free [Ca] and should improve the reliability of calculated free [Ca] in more complex solutions. Briefly, free [Ca] in Ca-EGTA solutions is measured with a Ca electrode, bound Ca is calculated, and Scatchard and double-reciprocal plots are resolved for the total [EGTA] and the apparent Ca-EGTA association constant (K'Ca) in the solutions used. The free [Ca] is then recalculated using the determined parameters, giving a more accurate knowledge of the free [Ca] in these solutions and providing an accurate calibration curve for the Ca electrode. These solutions can then be used to calibrate other Ca electrodes (e.g., Ca microelectrodes) or the calibrated Ca electrode can be used to measure free [Ca] in solutions containing multiple metal ligands. This method allows determination of free [Ca], K'Ca, and total [EGTA] in the actual solutions used regardless of pH, temperature, or ionic strength. It does not require accurate knowledge of K'Ca or EGTA purity and circumvents many potential errors due to assumption of binding parameters. K'Ca was found to be 2.45 +/- 0.04 X 10(6) M-1 in 100 mM KCl, 10 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid, and 1 mM EGTA at pH 7.00 and 23 degrees C. Total [EGTA] varied with supplier but was always less than quoted.

Calcium↗
PubMed1981

Cardiac contractility and sarcolemmal calcium binding in several cardiac muscle preparations.

Striking correlations are found between cardiac contractility and Ca2+ binding to isolated cardiac sarcolemma in rabbit, neonatal rat, and frog ventricular tissue. Deviations from this correlation are seen in the adult rat ventricle and rabbit atrium. The observation of this correlation in the three former tissues and under various ionic conditions suggests that this correlation is not coincidental and that Ca2+ bound to the cardiac sarcolemma is of major importance in the control of myocardial contractility. The data are consistent with a functional Ca2+-induced Ca2+ release system in the sarcoplasmic reticulum (SR) of all the tissues (which is controlled by Ca2+ entry from sarcolemmal sites), with the adult rat ventricular and rabbit atrial SR Ca2+ release being much more sensitive to CA2+. It is suggested that the frog, neonatal rat, and rabbit ventricles depend more directly on the entry of Ca2+ from sarcolemmal sites for the control of tension development, whereas the adult rat ventricle and rabbit atrium depend to a greater extent on CA2+ released from the SR.

Age Factors↗
PubMed1980

Sodium-calcium exchange and sidedness of isolated cardiac sarcolemmal vesicles.

The sidedness of isolated rabbit cardiac sarcolemmal vesicles was studied by observing the effects of several permeability-increasing agents on measurements of the amount of sialic acid released by neuraminidase, specific ouabain binding, and K+-phosphatase and (Na+ + K+)-ATPase activities. The results suggest that the vesicles are sealed and are about 80% right-side-out. Na+-Ca2+ exchange exhibited by these vesicles could be attributed to the sarcolemma rather than to some contaminating organelle. Ca2+ uptake was stimulated by preloading the vesicles with NaCl (and not KCl). Increasing the external [Na+] induced a rapid Ca2+ loss, which could not be mimicked by K+, Li+, Rb+ or choline+. The Nai+-dependent Ca2+ uptake was inhibited by certain cations: Cd2+ > La3+ > Y3+ > Mn2+ > Co2+ > Mg2+. The Nai+-dependent Ca2+ influx was enhanced by an inside positive membrane potential and inhibited by an inside negative membrane potential. Potentials were induced by a K+-valinomycin system.

Animals↗
PubMed1980

Binding of Ca2+ and Na+ to sarcolemmal membranes: relation to control of myocardial contractility.

Studies are presented that relate the calcium binding of isolated sarcolemmal membranes to myocardial contractility. The contractile strength of the perfused rabbit interventricular septum as a function of perfusate calcium concentration is compared with calcium bound to isolated rabbit sarcolemma at the same calcium concentrations. If the calcium-binding incubation medium contains 140 mM sodium chloride, the shapes of these two curves are identical. In another experiment, sarcolemmal calcium-binding data are compared with the results of Tillisch et al. (J. Mol. Cell. Cardiol. 11:137-148, 1979), who studied the response of rabbit papillary muscles to changes in the perfusate sodium concentration. Again a positive correlation between calcium bound and force developed is obtained. The results of experiments, using the isotope 22Na on the binding of sodium to isolated sarcolemmal membranes were subjected to Scatchard analysis. This revealed a single type of sodium receptor. The affinity constant for sodium binding is 110 M-1 and calcium appears to behave as a competitive inhibitor of sodium binding. The experimental results strongly imply a quantitative relationship between sarcolemmal calcium binding and myocardial contractility.

Animals↗
PubMed1979

Isolation and characterization of cardiac sarcolemma.

A procedure was developed for the isolation of cardiac sarcolemmal vesicles. These vesicles are enriched about ten-fold (with respect to the tissue homogenate) in K+-stimulated p-nitrophenylphosphatase, (Na+ + K+)-ATPase, 5'-nucleotidase activities and sialic acid content, all of which are believed to be components of the sarcolemma. The sarcolemma of tissue culture cardiac cells were radioiodinated and the distribution of this radioiodine paralleled the distribution of the other membrane markers above. There was very little contamination of the sarcolemmal fraction by sarcoplasmic reticulum (as judged by Ca2+-ATPase and glucose-6-phosphatase activities) or inner mitochondrial membranes (as judged by succinate dehydrogenase activity). There may, however, be some contamination by outer mitochondrial membranes (as judged by monoamine oxidase and rotenone-insensitive NADH cytochrome c reductase activities) which have rarely been monitored in cardiac sarcolemmal preparations. The purity of this preparation is good when compared with other cardiac sarcolemmal preparations. This preparation should be very useful in studying the roles of the cardiac sarcolemma (e.g. in excitation contraction coupling and Ca2+ binding).

4-Nitrophenylphosphatase↗
PubMed1979

Uncoupling cation effects on cardiac contractility and sarcolemmal Ca2+ binding.

Studies have been performed correlating the effects of cationic uncouplers on intact cardiac muscle and on Ca2+ bound to isolated cardiac plasma membranes. Sarcolemmal vesicles were isolated from neonatal rat hearts. Ca2+ binding experiments were performed and Scatchard plot analysis indicated two classes of Ca2+ binding sites. The ability of certain cations to displace Ca2+ bound to these vesicles was measured. The effects of these same cations on the tension developed by neonatal rat papillary muscles and on the Ca2+ content of tissue culture cells (from neonatal rat heart) were measured. The results show that a) the selectivity sequence (Y3+ greater than Nd3+ greater than La3+, Cd2+ greater than Co2+ greater than Mg2+) of sarcolemmal Ca2+ binding sites is the same as the effective uncoupling sequence; b) the amount of Ca2+ bound at these sites (approximately 700 micromol/kg wet wt) is more than adequate to support tension development; c) the dependence of sarcolemmal Ca2+ binding at low-affinity sites and tension development on [Ca2+]o is essentially the same. It is then reasonable to propose that the Ca2+ bound to these sarcolemmal sites plays an important role in controlling the amount of Ca2+ available to the myofilaments and thus myocardial contractility.

Animals↗
PubMedDate not supplied

Bay K 8644 increases resting Ca2+ spark frequency in ferret ventricular myocytes independent of Ca influx: contrast with caffeine and ryanodine effects.

Bay K 8644, an L-type Ca2+ channel agonist, was shown previously to increase resting sarcoplasmic reticulum (SR) Ca2+ loss and convert post-rest potentiation to decay in dog and ferret ventricular muscle. Here, the effects of Bay K 8644 on local SR Ca2+ release events (Ca2+ sparks) were measured in isolated ferret ventricular myocytes, using laser scanning confocal microscopy and the fluorescent Ca2+ indicator fluo-3. The spark frequency under control conditions was fairly constant during 20 s of rest after interruption of electrical stimulation. Bay K 8644 (100 nmol/L) increased the spark frequency by 466+/-90% of control at constant SR Ca2+ load but did not change the spatial and temporal characteristics of individual sparks. The increase in spark frequency was maintained throughout the period of rest. The increase in Ca2+ spark frequency induced by Bay K 8644 was not affected by superfusion with Ca2+-free solution (with 10 mmol/L EGTA) but was suppressed by the addition of 10 micromol/L nifedipine (which by itself did not alter resting Ca2+ spark frequency). This suggests that the effect of Bay K 8644 on Ca2+ sparks is mediated by the sarcolemmal dihydropyridine receptor but is also independent of Ca2+ influx. Low concentrations of caffeine (0.5 mmol/L) increased both the average frequency and duration of sparks. Ryanodine (50 nmol/L) increased the spark frequency and also induced long-lasting Ca2+ signals. This may indicate long-lasting openings of SR Ca2+ release channels and a lack of local SR Ca2+ depletion. In lipid bilayers, Bay K 8644 had no effect on either single-channel current amplitude or open probability of the cardiac ryanodine receptor. It is concluded that Bay K 8644 activates SR Ca2+ release at rest, independent of Ca2+ influx and perhaps through a functional linkage between the sarcolemmal dihydropyridine receptor and the SR ryanodine receptor. In contrast, caffeine and ryanodine modulate Ca2+ sparks by a direct action on the SR Ca2+ release channels.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗