Biomedical subjects

K G Morgan

Publications and source records attributed to K G Morgan.

At least 109 records · Page 6Linked to original sources

PubMed1986

The effects of amrinone and related drugs on intracellular calcium in isolated mammalian cardiac and vascular smooth muscle.

The effects of amrinone and related agents were studied in mammalian cardiac and vascular smooth muscle preparations loaded intracellularly with aequorin, a bioluminescent calcium indicator that emits light when it combines with Ca++. In cat papillary muscles, the effects of amrinone on the amplitude and time course of the aequorin light signal, as they relate to changes in uptake and release of calcium from the sarcoplasmic reticulum and to changes in the sensitivity of the myofilaments to Ca++, were consistent with reports that, in positive inotropic doses, the predominant action of amrinone is to increase intracellular concentrations of cyclic AMP. However, amrinone also produced dose-related caffeine-like effects on the aequorin light signal, suggesting additional subcellular actions. The positive inotropic effects of amrinone were most pronounced at low frequencies of stimulation (0.01 to 0.25 Hz) and were nearly obliterated at more physiologic heart rates (0.5 to 1.25 Hz). Toxic doses of amrinone were associated with development of a Ca++-overload state characterized by the presence of afterglimmers, aftercontractions, and arrhythmias. Amrinone's relaxant effect on ferret aortic and portal vein strips was associated with a corresponding decrease in intracellular Ca++ levels, and minimally effective relaxant doses were 10 to 100 times lower than positive inotropic doses reported for ferret heart. The vascular effects of amrinone were different from other agents known to increase intracellular cyclic AMP concentrations, suggesting multiple subcellular actions.

Actin Cytoskeleton↗

PubMed1985

Responses of enzymatically isolated mammalian vascular smooth muscle cells to pharmacological and electrical stimuli.

A modified method for enzymatically isolating mammalian vascular smooth muscle cells has been developed and tested for ferret portal vein smooth muscle. This method produces a high proportion of fully relaxed cells and these cells appear to have normal pharmacological responsiveness. The ED50 values for both alpha stimulation and potassium depolarization are not significantly different in the isolated cells from those obtained from intact strips of ferret portal vein, suggesting that the enzymatic treatment does not destroy receptors or alter the electrical responsiveness of the cells. It was also possible to demonstrate a vasodilatory action of papaverine, nitroprusside and adenosine directly on the isolated cells indicating that the pathways involved are intact in the isolated cells. This method should be of considerable usefulness, particularly in combination with the new fluorescent indicators and cell sorter techniques which require isolated cells.

Animals↗

PubMed1985

Alterations of excitation-contraction coupling by platelet-derived growth factor in enzymatically isolated and cultured vascular smooth muscle cells.

We studied stimulus-specific alterations of the excitation-contraction coupling pathway in freshly isolated contractile and subcultured non-contractile vascular smooth muscle cells. Using the calcium indicator aequorin, we detected physiological increases in cytoplasmic free calcium [( Ca2+]i) in subcultured smooth muscle cells subjected to angiotensin or 33 mM potassium depolarization. These increases were qualitatively identical to those previously measured in intact vascular strips. Platelet-derived growth factor (PDGF) induced a slow, sustained [Ca2+]i increase when applied to the subcultured smooth muscle cells at low picomolar concentrations. Freshly isolated, contractile vascular smooth muscle cells, prepared by a novel technique, exhibited a slow shortening of 20% of resting length in response to PDGF. PDGF also markedly potentiated smooth muscle cell shortening in response to an ED50 dose of phenylephrine. This effect was PDGF concentration dependent. The time course of shortening induced by PDGF alone was consistent with the time course of the PDGF-induced [Ca2+]i increase in the cultured smooth muscle cells. These data suggest that agonists which induce [Ca2+]i changes in contractile smooth muscle cells may retain this ability with respect to cultured smooth muscle cells. PDGF, a peptide mitogen for proliferative smooth muscle cells, may also serve to modulate vascular tone by modestly raising [Ca2+]i in contractile smooth muscle cell and, therefore, sensitizing the cells to alpha adrenergic agonists.

Aequorin↗

PubMed1985

Malignant epithelioid schwannoma of superficial soft tissue? A case report with immunohistology and electron microscopy.

Malignant epithelioid schwannoma is a well-defined entity. However epithelioid tumours arising other than in a nerve trunk, in a benign nerve sheath tumour or in neurofibromatosis are problematical. We describe a case of a nodular tumour of the cheek which recurred twice without metastasis. Light microscopy showed an amelanotic epithelioid tumour. Immunohistology showed positive staining for vimentin and S100 protein and electron microscopy showed distinctive appearances suggestive of schwannoma. We discuss the differential diagnosis of the tumour, compare it with reported tumours resembling both melanoma and schwannoma and conclude that this may be a further example of the distinctive tumour reported as "malignant epithelioid schwannoma of superficial soft tissues'.

Aged↗

PubMed1985

Calcium-force relationships as detected with aequorin in two different vascular smooth muscles of the ferret.

The bioluminescent calcium indicator aequorin was chemically loaded into isolated strips of ferret portal vein and ferret aorta. Aequorin light emission (a function of [Ca2+]i) was recorded simultaneously with tension. Assuming an [Mg2+]i of 0.5 mM, [Ca2+]i was 1.8 X 10(-7) M in the unstimulated portal vein at 22 degrees C where there was negligible resting tone. In contrast, in the unstimulated aorta at 22 degrees C where there was significant basal tone, the [Ca2+]i was 2.7 X 10(-7) M. In both portal vein and aorta, potassium depolarization caused a monophasic rise in intracellular Ca2+ in parallel with the rise in tension, whereas phenylephrine caused an initial spike of light during the period of the force development which then fell to a much lower plateau level during the period of force maintenance. Calcium-force curves were generated by plotting calibrated aequorin light against force while intracellular [Ca2+] was made to change either by increasing degrees of potassium depolarization or decreasing extracellular [Ca2+]. The steady-state calcium-force curve in the presence of phenylephrine was shifted to the left of the curve in the presence of potassium depolarization in both the portal vein and aorta. In the aorta there was a counter-clockwise hysteresis in the calcium-force relationship. In contrast, in the portal vein there was no demonstrable hysteresis, indicating that the apparent change in calcium sensitivity of the contractile apparatus in the presence of phenylephrine must be caused by a second messenger other than calcium.

Aequorin↗

PubMed1985

Intracellular electrical activity of muscularis mucosae of the dog stomach.

Mechanical and intracellular electrical activity was recorded simultaneously in vitro from smooth muscle of the muscularis mucosae of the canine antrum. The intracellularly recorded membrane potential averaged -51 +/- 1.4 mV (mean +/- SE). Spontaneous electrical activity consisted of spike-shaped potentials that were 20-40 mV in amplitude. The rate of rise of the spike potential was slow (less than 0.2 V/s) and the half-time duration was long (0.5-5.0 s). Phasic contractions were often but not always coupled with spike potentials. Ion substitution studies suggested that the spike potential had a greater dependence on Na+ than on Ca2+. Field stimulation of intramural nerves hyperpolarized the membrane potential and abolished spikes or reduced their amplitude and frequency. These changes were associated with a reduction in tone and phasic contractile activity. The response to stimulation of inhibitory nerves was mimicked by epinephrine, neurotensin, and vasoactive intestinal polypeptide. The resistance to adrenergic blocking agents ruled out the possibility of norepinephrine as the transmitter. The tetrodotoxin sensitivity of the response to neurotensin suggests that neurotensin acts indirectly through the inhibitory nerves. Mimicry between the action of applied vasoactive intestinal polypeptide (VIP) and field stimulation provides support for the hypothesis that VIP may be an inhibitory neurotransmitter. These studies indicate that smooth muscle in the canine gastric muscularis mucosae generates spontaneous electrical and mechanical activity and receives a noncholinergic, nonadrenergic inhibitory innervation.

Action Potentials↗

PubMed1985

Cellular Ca2+ monitored by aequorin in adenosine-mediated smooth muscle relaxation.

We studied the effect of adenosine on cytoplasmic ionized calcium-force relationships in vascular smooth muscle (VSM) and determined the dose dependence of the observed effects. The bioluminescent protein aequorin was used as an index of cytoplasmic ionized calcium and was chemically loaded into ferret portal vein strips. The VSM strips were contracted with 33 mM potassium (K+), 5 X 10(-6) M phenylephrine (PE), or electrical stimulation. Force and aequorin light, i.e., cytoplasmic ionized calcium, were simultaneously recorded. Adenosine pretreatment (3.7 X 10(-6) M) reduced both force and light responses in contractures with K+, PE, or electrical stimulation. In contrast, the addition of adenosine during PE or K+ contractions decreased force without a change in light. Dose-response curves for the effects of adenosine on K+ contractures indicated that at low doses adenosine decreases force and cytoplasmic ionized calcium but that at high concentrations (greater than 3.7 X 10(-6) M) adenosine increases light and apparently relaxes VSM by desensitizing the myofilaments to cytoplasmic ionized calcium.

Adenosine↗

PubMed1984

Calcium and cardiovascular function. Intracellular calcium levels during contraction and relaxation of mammalian cardiac and vascular smooth muscle as detected with aequorin.

Calcium ion (Ca++) plays a central role in excitation-contraction coupling of both cardiac and vascular smooth muscles and have important functional interactions with other cations, including sodium, potassium, and magnesium. Ca++ transients associated with contraction-relaxation cycles of the heart and vasculature can now be recorded directly by use of aequorin, a bioluminescent protein that emits light when it combines with Ca++. After microinjection or chemical loading of aequorin into the sarcoplasm, light output provides an index of intracellular [Ca++]. In cardiac muscle, intracellular Ca++ increases more quickly than tension and decreases toward basal levels by the time peak tension is reached. The calcium transients of working myocardium in both human subjects and other mammalian species appear to be dominated by the release and uptake of Ca++ from intracellular stores under all conditions studied. Drugs and disease states produce marked changes in the amplitude and time course of the Ca++ transient and the corresponding contractile response. In vascular smooth muscle, there are stimulus-specific patterns in intracellular Ca++ associated with tonic contractions. Although Ca++ is related to tension development, the relationship appears to be more complex than that in cardiac muscle. As a result, tension development cannot be used as an index of free Ca++ levels in vascular smooth muscle. Selection of the most effective therapy to reverse a tonic contraction in states of spasm or hypertension may depend on the specific stimulus that caused the increased tone.

Aequorin↗

PubMed1984

A chemical procedure for loading the calcium indicator acquorin into mammalian working myocardium.

The bioluminescent calcium indicator aequorin was successfully loaded into mammalian working myocardium of ferrets by a chemical procedure which makes the cells reversibly hyperpermeable through exposure to Ethylenebis-( oxyethylenenitrilo ) tetraacetic acid (EGTA). After undergoing the loading procedure, developed tension at Lmax was 103 +/- 26% of the control, which indicates that the muscles regained normal function. The configurations of the aequorin signals (i.e., calcium transients) and their responses to drugs were the same as reported after microinjection of aequorin. The peak of the Ca++ transient determined by the method of fractional luminescence at 3s intervals of stimulation, 2.5mM [Ca++]o, 22.5 degrees C was 1.1 microM; this value is similar to that reported for microinjection. These results indicate that the chemical loading procedure is a useful alternative to microinjection for loading aequorin into mammalian working myocardium.

Aequorin↗

PubMed1984

Stimulus-specific patterns of intracellular calcium levels in smooth muscle of ferret portal vein.

Aequorin was loaded into cells of the ferret portal vein in order to follow intracellular Ca2+ levels during smooth muscle contraction. In response to a single d.c. pulse, the aequorin signal reaches a peak during the rising phase of the evoked force transient and begins to rapidly fall while force is still rising. In response to the addition of phenylephrine to the bathing solution, the aequorin signal rises rapidly to a peak while force is still rising but then falls rapidly to a lower level from which it declines more slowly, staying above base-line levels as long as force is maintained. In response to the elevation of K+ concentration in the bathing solution, light and force rise together and the elevated light level is maintained as long as is force. With increasing concentrations of K+, force increases up to a concentration of 50-60 mM but light increases up to 90 mM, suggesting that at a concentration of 50-60 mM-K+, the Ca2+ concentration may be saturating with respect to a site of action on the contractile apparatus. During the period of force maintenance, phenylephrine produces a larger ratio of force to light than does K+ depolarization. The maintenance of force in the presence of either phenylephrine or elevated K+ requires an elevation of intracellular Ca2+ levels above base-line values. These results suggest that phenylephrine can increase the effectiveness of Ca2+ on the contractile apparatus.

Aequorin↗

PubMed1984

Alteration of cytoplasmic ionized calcium levels in smooth muscle by vasodilators in the ferret.

Aequorin was used as an indicator of cytoplasmic Ca2+ levels during the action of vasodilators in potassium-depolarized strips of ferret portal vein. Moderate concentrations of isoprenaline produced either no change or an increase in cytoplasmic Ca2+ levels during smooth muscle relaxation. Only in the presence of very high concentrations of isoprenaline (greater than 10(-4) M) was a decrease in intracellular Ca2+ levels detectable. Both papaverine and forskolin also caused relaxations of the muscle while cytoplasmic Ca2+ levels were observed either not to change or to increase. When the muscles were relaxed either by decreasing the calcium concentration in the bathing medium or by the addition of sodium nitroprusside, light and force fell together. The ratio of force to light was greater when the muscle was relaxed with calcium depletion than with the addition of sodium nitroprusside, indicating that sodium nitroprusside was relaxing the muscle by more than just decreasing cytoplasmic Ca2+ levels. These results indicate that not all vasodilators cause a lowering of cytoplasmic Ca2+ levels and indicate that, in intact cells, agents associated with increases in cytoplasmic cyclic AMP levels can cause an uncoupling of calcium-force relations.

Aequorin↗

PubMed1983

Comparison of membrane electrical activity of cat gastric submucosal arterioles and venules.

Intracellular electrical recordings were made from arterioles and venules of the cat gastric submucosa. Spontaneous rhythmic fluctuations of the membrane potential were recorded in 54% of the venular preparations. Arteriolar cells showed no spontaneous activity. Excitatory junction potentials were recorded in arterioles but not venules after single shocks to the perivascular nerves. The amplitude of the excitatory junction potential was decreased in the presence of alpha-blockers. Repetitive stimulation of the perivascular nerve caused a biphasic electrical response of venular smooth muscle cells. The depolarizing component was decreased by alpha-adrenergic blockade and the hyperpolarizing component by beta-blockade. Venules contracted in response to smaller depolarizations than did arterioles. The voltage threshold for contraction of venular cells was similar to that for arteriolar cells but the venular cells were significantly more depolarized at rest than were the arteriolar cells. The difference in resting potential provides an explanation for the difference in sensitivity to electrical input.

Action Potentials↗

PubMed1983

Pentagastrin potentiates nonadrenergic inhibitory neuromuscular transmission in orad stomach of the dog.

Mechanical and intracellular electrical activities were recorded separately from circular muscle of the orad corpus of the canine stomach. In normal Krebs solution, transmural electrical nerve stimulation produced excitatory and inhibitory responses. The excitatory response was blocked by atropine; the inhibitory response was unaffected by phentolamine or propranolol alone or in combination. Both responses were blocked by tetrodotoxin. In the presence of pentagastrin, the inhibitory response to electrical nerve stimulation was potentiated in a dose-dependent manner. The threshold concentration for this potentiating effect was 3 X 10(-11) M. Intracellular recordings showed that hyperpolarization of the resting membrane potential in response to inhibitory nerve stimulation was greater in the presence of pentagastrin than in its absence. These data suggest that, in addition to a direct stimulatory effect on the muscle, pentagastrin also potentiates the effect of ongoing activity of intramural inhibitory nerves. This latter effect is only apparent when the inhibitory neural network is active. Thus, the in vivo effect of pentagastrin on motor activity of the corpus will depend on the degree of ongoing activity of intramural inhibitory nerves.

Animals↗

PubMed1983

Electrophysiological differentiation of alpha-receptors on arteriolar smooth muscle.

The effects of clonidine (a prototype of an alpha 2-agonist) and phenylephrine (a prototype of an alpha 1-agonist) on intracellularly recorded electrical activity and vessel size of feline submucosal arterioles were compared. Phenylephrine constricts the vessels and causes a depolarization and the initiation of oscillations of the membrane potential. These oscillations occasionally give rise to spike potentials. In contrast, clonidine produces no significant depolarization of the resting potential in spite of the simultaneous initiation of contraction. Neurally induced depolarizations (excitatory junction potentials) are not blocked and are sometimes augmented by the nonselective alpha-blocker phentolamine even though the depolarization induced by norepinephrine is blocked by phentolamine. Excitatory junction potentials are antagonized by the alpha 1-blocker prazosin. The contraction caused by clonidine is blocked to a greater degree by yohimbine (a relatively selective alpha 2-blocker) than by prazosin. The contraction caused by phenylephrine is blocked to a greater degree by prazosin than by yohimbine. These data indicate that phenylephrine and clonidine act by different mechanisms and, taken together with previous studies, suggest that alpha 1- and alpha 2-stimulation utilize different excitation-contraction coupling mechanisms.

Animals↗

PubMed1982

Vascular smooth muscle: the first recorded Ca2+ transients.

The bioluminescent calcium indicator aequorin was successfully loaded into vascular smooth muscle cells of Amphiuma tridactylum by either microinjection or a new method which makes the cells reversibly hyperpermeable. Both gave similar results; however, the latter method produced larger signals. Vasoconstrictors produced a sustained contraction and a light (calcium) response consisting of two components: a large transient followed by a smaller, sustained response. Electrical stimulation produced a light transient that was much briefer than the contraction. These results suggest that tension can be maintained in smooth muscle in the presence of lower calcium levels than those present during force development.

Aequorin↗

PubMed1982

Innervation of the muscularis mucosa in the canine stomach and colon.

A preliminary report is presented of studies into the innervation of the canine muscularis mucosa. In vitro experiments have investigated the mechanical response to electrical stimulation and the release of gut peptides known to modulate gastrointestinal function. Exogenous substance P increased tone and amplitude of phasic contractions while neurotensin and vasoactive intestinal polypeptide (VIP) relaxed the muscularis mucosa. Immunoreactive VIP was released into the superfusate during transmural electrical stimulation of the antrum while VIP and substance P were present in superfusate from the colon. The significance of these findings has been discusses briefly.

Animals↗

PubMed1981

The electrical basis for contraction and relaxation in canine fundal smooth muscle.

1. Mechanical and intracellular electrical activities were recorded simultaneously from canine fundal and antral smooth muscle preparations.2. Most fundal preparations displayed no spontaneous electrical or mechanical activity. The tissue had a space constant of 1.5 mm and a time constant of 189 msec and showed outward rectification in response to depolarizing current.3. Transmural nerve stimulation of fundal preparations demonstrated the presence of cholinergic excitatory and non-cholinergic, non-adrenergic inhibitory neural inputs to the tissue. The cholinergic nerve response consisted of a small, graded depolarization accompanied by a slow graded contraction; the inhibitory nerve response consisted of a graded hyperpolarization accompanied by a slow relaxation.4. The excitatory fundal nerve response was abolished or greatly diminished by D(600) and Mn(2+). D(600) and Mn(2+) also decreased basal tone. The inhibitory nerve response was unaffected by either agent.5. The excitatory nerve response in the fundus was contrasted with the excitatory nerve response in the antrum. In the fundus, stimulation of cholinergic motor nerves produced a depolarization which always produced a contraction. In the antrum, stimulation of cholinergic motor nerves between action potentials produced graded depolarizations of antral cells; however, there were no associated contractions. Stimulation of cholinergic motor nerves during spontaneous action potentials increased the amplitude and duration of the plateau phase of the action potential; this was associated with an augmentation of the spontaneous contractions.6. Voltage-tension curves were determined for antral and fundal preparations using K(+) depolarization as a means of controlling membrane potential. Antral preparations displayed a voltage threshold for contraction at a membrane potential approximately 30 m V positive to the resting potential. In cntrast, fundal resting potentials were at or more positive than their voltage thresholds.7. These differences in electromechanical coupling provide an explanation for the marked differences in the responses of fundal and antral smooth muscles to nerve stimulation and account for their physiologic function in vivo.

Acetylcholine↗

PubMed1980

Use of aequorin to study excitation--contraction coupling in mammalian smooth muscle.

Considerable information is available for intact skeletal and cardiac muscle cells on the role of changes in intracellular Ca2+ (Cai2+) levels during excitation-contraction coupling. Cai2+ levels seem to be similarly important in smooth muscle. However, only two published reports describe the use of indicators to directly measure [Cai2+] in intact smooth muscle cells. One described the use of murexide in molluscan smooth muscle, the other the use of aequorin in single amphibian cells. Their results differed: very long calcium transients were described for the molluscan muscle, compared with very brief ones for the amphibian cells. We present here the first description of the relationship between calcium transients and contraction of whole, mammalian smooth muscle injected with aequorin. The Ca2+ transient we observed has a slow rise, a long duration, and there was no delay between the peak of the Ca2+ transient and the onset of force. This contrasts with the amphibian findings but resembles the molluscan data. We observed a delay between the onset of the increase in [Cai2+] and the onset of the rise in tension which might be explained, at least in part, by the presence of a threshold [Cai2+] for concentration.

Action Potentials↗