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Biomedical subjects

M D Stern

Publications and source records attributed to M D Stern.

At least 109 records · Page 6Linked to original sources
PubMed1985

Bounds on the estimation of calcium by aequorin luminescence in the presence of inhomogeneity.

The photoprotein aequorin, which emits light as a nonlinear function of calcium concentration, is often used to measure intracellular calcium. In the presence of inhomogeneities or fluctuations of calcium concentration, the nonlinearity results in discrepancies between mean calcium concentration estimated from average aequorin light and the true mean. It is usually assumed that the error is an overestimation, but in the presence of large calcium fluctuations, errors of either direction are possible. Here we show that for aequorin to overestimate the mean calcium, the point in the calcium-light plane representing the true mean calcium and measured mean aequorin light must lie in the convex envelope of that segment of the aequorin response curve that lies between the minimum and maximum values of fluctuating calcium, and must lie above the curve. By explicitly constructing this region, we derive a quartic equation that gives the largest measured calcium for which aequorin can be assumed to give an overestimate, as a function of the maximum calcium fluctuation. In particular, if calcium fluctuations do not exceed 1 mM, aequorin measurements below 7.25 microM may be assumed to overestimate the true mean calcium.

Aequorin↗
PubMed1985

Fluctuations in intracellular calcium concentration and their effect on tonic tension in canine cardiac Purkinje fibres.

Ca2+-activated aequorin luminescence and tension were measured in dog Purkinje fibres during twitches and during the increase in resting force produced by exposure of the fibres to a low-Na+ solution after 3 min without external K+. Over the restricted range which could be examined, the relation between tension and 'mean' aequorin luminescence (luminescence filtered at 0.2 Hz) was approximately linear during the development and maintenance of contracture. For a given level of force, the mean aequorin luminescence during contracture was up to 20 times greater than the peak luminescence during the twitch. Noise analysis of aequorin luminescence and tension during contracture indicated the presence of periodic fluctuations, with a predominant frequency in the range 1-4 Hz. Ryanodine (1 microM) or caffeine (10 mM) abolished the fluctuations in luminescence and tension and made the relation between tension and mean aequorin luminescence much steeper. A mathematical model, the key feature of which is periodicity in the asynchronous occurrence of spatially localized regions of relatively high [Ca2+], reproduces the experimental data derived from contractures. From the model analysis, we infer that tonic tension is produced by recruitment of increasing numbers of regions of high [Ca2+], rather than by homogeneous graded activation. These results indicate that during contracture or 'tonic tension', intracellular [Ca2+] is not at steady state, but rather undergoes large, asynchronous spatio-temporal fluctuations. Thus the assumptions that intracellular [Ca2+] is at steady state or homogeneous during tonic tension are not valid.

Action Potentials↗
PubMed1985

Direct observation of the "oxygen paradox" in single rat ventricular myocytes.

By phase contrast microscopy with video length tracking, we followed the sequence of morphological changes in individual isolated rat ventricular myocytes during anoxia followed by reoxygenation. Cells appeared normal during early anoxia. After a duration of anoxia T1, which varied from 17-47 minutes in different cells, each cell abruptly contracted an average of 33% in length to an inert rectangular form presumed to be a rigor state. Cells which were reoxygenated before the onset of rigor showed normal morphology and an unchanged extent of shortening on field stimulation, compared to control. Cells that were reoxygenated after a time in the rigor state, T2, either partially recovered to a shortened rectangular form capable of stimulated twitches or rounded up rapidly to a disordered hypercontracture form. The distribution of T1 was the same for cells which recovered and which hypercontracted. In contrast, the outcome of reoxygenation depended markedly on T2: all cells that were reoxygenated after less than 10 minutes of rigor recovered function, whereas all cells that spent more than 20 minutes in rigor hypercontracted when reoxygenated. The hypercontracture appears to be the cellular analog of the "oxygen paradox" in whole hearts. Its occurrence is reliably related to duration of rigor state but not to duration of hypoxia, because of marked cellular variability in the time of onset of rigor.

Adenosine Triphosphate↗
PubMed1985

Effect of supplemental niacin or niacinamide and soybean source on ruminal bacterial fermentation in continuous culture.

Effects of niacin or niacinamide in diets containing either soybean meal, raw whole soybeans or whole soybeans extruded at 132 and 149 C on ruminal bacterial fermentation were examined with a dual-flow continuous culture system. In Exp. 1, soybean sources each provided 50% of total crude protein in diets comprised of 52% concentrate mix, 36% corn silage and 12% alfalfa hay (dry-matter basis). Each diet was supplemented with 0 or 100 mg/kg niacin. Niacin supplementation increased (P less than .05) total nonstructural carbohydrate digestibility and lowered (P less than .05) butyrate concentration. There was also an increase (P less than .10) in amino acid effluent flow from 8,413.3 to 8,665.3 mg/d with addition of niacin to the diet. In Exp. 2, diets were supplemented with 0 or 100 mg/kg of niacin or niacinamide. The total mixed diet was comprised of 60% concentrate mix, 20% corn silage and 20% alfalfa hay (dry matter basis). Acid detergent fiber and cellulose digestibilities and total amino acid effluent flow were higher (P less than .10) with niacinamide supplementation. Niacin or niacinamide had no effect on dry matter and organic matter digestibilities, ammonia-N, total VFA concentration or crude protein degradation. Contrary to results found in other studies, niacin or niacinamide supplementation had no effect on bacterial protein synthesis.

Animals↗
PubMed1985

Protein degradation in rumen and amino acid absorption in small intestine of lactating dairy cattle fed heat-treated whole soybeans.

Four lactating Holstein cows fitted with rumen cannulae and T-type cannulae in proximal duodenum and terminal ileum were used to measure protein degradation in the rumen and amino acid flow and absorption in the small intestine. Soybean meal, whole soybeans, and whole soybeans extruded at 132 and 149 degrees C provided 50% of the protein in diets that contained 51% grain, 36% corn silage, and 13% alfalfa hay (dry matter). Spot samples of digesta were collected from duodenum and ileum during 96 h, and lanthanum was an indigestible marker to estimate flow and digestibility of nutrients. With diaminopimelic acid as a microbial marker, apparent degradations of dietary crude protein in the rumen were 73, 80, 66, and 60% for diets containing soybean meal, whole soybeans, and whole soybeans extruded at 132 and 149 degrees C. Because of the extensive degradation of protein with the raw soybean diet, less total amino acids reached the duodenum, and because of decrease of availability, absorption from the small intestine (g/day) was lowest with this diet. Feeding diets containing extruded whole soybeans increased availability of total essential amino acids in the small intestine compared with diets containing soybean meal and whole soybeans. Absorption from the small intestine (g/day and percent entering) of individual amino acids was generally higher for extruded whole soybean diets.

Amino Acids↗
PubMed1985

Influence of methionine hydroxy analog and DL-methionine on rumen protozoa and volatile fatty acids.

Three lactating Holstein cows with rumen cannulae were used in a 3 X 3 Latin square arrangement of treatments to study effects of methionine source on ciliated protozoa numbers and volatile fatty acid concentrations in the rumen. Cows were fed total mixed diets twice daily of 60% grain mix, 21% corn silage, and 19% alfalfa hay (dry basis). Methionine hydroxy analog, DL-methione, and sodium sulfate were added on an equal sulfur basis. Experimental periods were 14 d with four rumen samples taken daily on d 13 and 14. Total ciliated protozoa were higher when cows were fed DL-methionine than when fed methionine hydroxy analog or sodium sulfate. Ruminal concentrations of butyric acid and isobutyric acid were higher for the DL-methionine treatment than for analog or sodium sulfate treatments. Isovaleric acid concentrations were also higher in fluid from cows fed methionine compared with cows fed the analog.

Animals↗
PubMed1985

Spontaneous myocardial calcium oscillations: overview with emphasis on ryanodine and caffeine.

All mammalian cardiac preparations exhibit the capacity for periodic spontaneous Ca2+ release from the sarcoplasmic reticulum (SR) (Ca2+ oscillations). The occurrence of such oscillations in unstimulated preparations and their periodicity depend on the species and the Ca2+ load on the cell. When the spontaneous frequency of these oscillations exceeds the rate of external simulation, they appear between stimulated contractions and impart a variable Ca2+-dependent component of diastolic tonus and a propensity for extrasystoles and arrhythmias to occur; these diastolic oscillations can also affect systolic function as well. Although enhancing the spontaneous frequency of Ca2+ release, caffeine depresses the oscillation amplitude, whereas ryanodine suppresses both frequency and amplitude. Detailed studies of oscillation characteristics and of the different effects of caffeine and ryanodine on them may provide an understanding of and may be useful for modeling SR Ca2+ uptake and release in intact preparations.

Alkaloids↗
PubMed1984

Radioreceptor assay for serum levels of cetamolol, a new beta-adrenoceptor antagonist.

Cetamolol hydrochloride, viz 2-[2-[3-(1,1-dimethylethyl)amino-2-hydroxy-propoxy]phenoxy]-N- methylacetamide hydrochloride, is a potent beta-adrenoceptor antagonist with moderate cardioselectivity and intrinsic sympathomimetic activity. In support of ongoing clinical trials, a radioreceptor assay has been developed to measure serum drug levels. In this assay, serum extracts containing cetamolol compete with [3H]-dihydroalprenolol for rat lung binding sites. This method which is sensitive to 1 ng/mL has been validated with respect to stability, blank, specificity, accuracy, within-assay precision, between-assay precision and parallelism. This method has been applied to a preliminary clinical study where serum drug levels were detectable up to 24 h after a single oral dose of 10 or 25 mg cetamolol hydrochloride.

Acetamides↗
PubMed1984

Protein degradation in the rumen and amino acid absorption in the small intestine of lactating dairy cattle fed various protein sources.

Four lactating Holstein cows fitted with a rumen cannula and T-type cannulae in the duodenum and ileum were used in an experiment with a 4 X 4 Latin square arrangement of treatments. Diets containing soybean meal (SBM), corn gluten meal (CGM), wet brewers grains (WBG) and distillers dried grains with solubles (DDG) were compared with regard to protein degradability in the rumen and amino acid flow and absorption in the small intestine. The test protein sources provided about 50% of the dietary protein. Spot samples of digesta were collected during a 96-h period and lanthanum (La) was used as an indigestible marker to estimate flow and digestibility of nutrients. Apparent organic matter digestibility in the rumen was 31 +/- 4.8%, while true digestibility based on measurements of microbial organic matter averaged 55 +/- 5.1%. Ruminal ammonia-nitrogen (NH3-N) concentrations (mg/100 ml) were 10.1, 5.6, 6.9 and 6.4 for the SBM, CGM, WBG and DDG diets, respectively. Using diaminopimelic acid as a microbial marker, protein degradation in the rumen was higher for the SBM diet (70%) than for the CGM (45%), WBG (52%) and DDG (46%) diets. Flow of amino acids to the small intestine was generally higher than amino acid intake for all diets. Apparent absorption of amino acids from the small intestine was 70, 77, 71 and 66% of the amino acids entering the duodenum for the SBM, CGM, WBG and DDG diets, respectively. Actual amounts absorbed (g/d) were lowest for the SBM diet. It was concluded that diets containing CGM, WBG or DDG will generally supply more total amino acids to the intestine than a diet containing SBM. Net amounts of amino acids available for absorption in the intestine were also higher for these diets because protein availability in the intestine was not impaired even though protein was more resistant to microbial breakdown in the reticulorumen.

Amino Acids↗
PubMed1983

Cellular calcium fluctuations in mammalian heart: direct evidence from noise analysis of aequorin signals in Purkinje fibers.

Indirect evidence suggests that fluctuations, or oscillations, in the intracellular free calcium concentration [( Ca2+]i) can occur spontaneously in intact cardiac preparations, but such [Ca2+]i fluctuations have never been demonstrated directly. We used the bioluminescent Ca2+-sensitive protein aequorin to detect fluctuations in the [Ca2+]i in canine cardiac Purkinje fibers. Noise analysis of the aequorin luminescence reveals prominent peaks of power density at frequencies of 1-4 Hz; these peaks become larger and shift to higher frequencies as the [Ca2+]i increases. Caffeine and ryanodine abolish the [Ca2+]i fluctuations, suggesting that Ca2+ release and uptake by the sarcoplasmic reticulum generate these events. When [Ca2+]i fluctuations are present, less tension is produced at any given level of mean aequorin luminescence. Thus, [Ca2+]i fluctuations will undermine attempts to relate [Ca2+]i and force in intact myocardium.

Animals↗
PubMed1983

Evaluation of a mathematical model of rumen digestion and an in vitro simulation of rumen proteolysis to estimate the rumen-undegraded nitrogen content of feedstuffs.

Twelve grain mixtures, one lucerne (Medicago sativa) hay and one maize silage which had been used in mixed diets for which dietary nitrogen undegraded in the rumen (UDN) had been estimated with duodenally-cannulated cows, were studied. Total N in the feeds was fractionated into pool A (N soluble in borate-phosphate buffer), pool B (total N--(pool A + pool C)) and pool C (acid-detergent-insoluble N or residual N after 24 h incubation in protease solution). N solubilization in protease solution containing 6.6 units/ml (substrate-saturating enzyme concentration) indicated the presence of subfractions in pool B, with different rates of solubilization. Such subfractions were not detectable from in situ, Dacron bag, estimates of N solubilization. UDN was estimated using a dynamic mathematical model and rate-constants obtained from N solubilization in protease solution or in situ. For three grain mixtures tested using the protease technique the model predicted UDN values of 7, 10 and 12% compared with values of 47, 66 and 59% estimated in vivo. The full range of experimental feeds was tested using the in situ technique and UDN values predicted by the model were used to derive UDN values for twelve mixed diets. The latter values were significantly but not closely correlated with those determined in vivo (r2 0.41, P less than 0.05). An attempt was made to simulate rumen proteolysis in vitro by choosing a protease enzyme concentration (0.066 units/ml) providing a proteolytic activity similar to that of whole rumen fluid. The experimental samples of feed were subjected to simulated rumen proteolysis for 18 or 48 h to resemble the mean retention times in the rumen for grain mixtures and roughages respectively. The residual N at the end of incubation was considered as an estimate of UDN. The UDN values estimated from simulated rumen proteolysis and those determined in vivo for twelve mixed diets were in close agreement (r2 0.61, P less than 0.01). Simulated rumen proteolysis can serve as a simple, rapid and sensitive method to estimate UDN in a variety of feedstuffs.

Animal Feed↗
PubMed1983

Scattered-light intensity fluctuations in diastolic rat cardiac muscle caused by spontaneous Ca++-dependent cellular mechanical oscillations.

Laser light scattered by nonstimulated rat cardiac muscle bathed in physiological saline containing a [Ca++] of 0.4-2.5 mM displays scattered-light intensity fluctuations (SLIF); the frequencies of both SLIF and resting force are Ca++ dependent. Direct inspection of these muscles by phase-contrast microscopy under incoherent illumination revealed the presence of spontaneous asynchronous cellular motions that are also Ca++ dependent. The physical properties of the scattered light are compatible with the hypothesis that SLIF are due to the diastolic motion, except for the dependence on scattering angle, which may be perturbed because the muscles are optically thick. To determine whether diastolic SLIF and motion are an intrinsic property of activated myofilaments, photon-counting auto-correlation of the scattered light was performed both in rat right-ventricular papillary muscles skinned with the detergent Triton X-100 (1%) and in muscles with intact membranes under conditions that alter cellular Ca++ fluxes. In skinned muscles activated over a range of Ca++ from threshold to maximum force production, neither SLIF nor asynchronous motion was observed when Ca++ was buffered to constant values. In intact muscles the frequency of SLIF and the amplitude of diastolic motion were (a) markedly increased by substituting K+ or Li+ for Na+ in the bath; (b) not altered by verapamil (1 microM); and (c) reversibly abolished by caffeine (greater than or equal to 10 mM). These properties are exactly those of mechanical oscillations that have been observed in isolated cardiac cell fragments, which are the result Ca++ oscillations caused by Ca++ release from the sarcoplasmic reticulum (SR). We infer that mechanical oscillations caused by spontaneous Ca++-induced Ca++ release from the SR occur in intact nonstimulated cardiac muscle even in the absence of Ca++ overload and are the principle cause of SLIF, and that myoplasmic [Ca++] in "resting" muscle is not in a microscopic steady state.

Animals↗
PubMed1983

Factors affecting disappearance of feedstuffs from bags suspended in the rumen.

Four experiments were conducted to examine the effect of various factors on in situ digestion of feedstuffs in the rumen of Holstein cows. In Exp. 1, the effect of various days, animals and animal diets on in situ digestion was evaluated. Measured disappearance of dry matter (DM) and nitrogen (N) from soybean meal (SBM) suspended in dacron bags was lowest when the host animal was fed a high grain diet. No effect due to animals, day of incubation or period of experimentation was observed. The effect of bag porosity and substrate particle size on DM and N disappearance from bags was studied in Exp. 2. Rip-stop nylon, a material with small pores, appeared to decrease the influx of digesting agents into bags and limited efflux of digested residues from bags when compared with bags made from dacron polyester material. Disappearance of DM and N from SBM and distillers grains (DG) in rip-stop nylon bags was greater when these feedstuffs were in the pulverized form rather than in the commercially processed form. In Exp. 3, dacron and Acropore bags, with pore sizes of 52 and 5 microns, respectively, were used to examine DM and N disappearance, in the rumen of cows, from total mixed diets containing various dietary N sources. Lower DM and N disappearance from Acropore bags was observed for each of the diets regardless of time spent in the rumen; much of the difference was established within the first hour. The effect of formaldehyde treatment on degradation of amino acids from SBM suspended in dacron bags in the rumen for 16 h was studied in Exp. 4. Increased resistance of SBM amino acids to degradation in the rumen was observed as a result of formaldehyde treatment levels as low as .3% by weight. A possible increase in protection of lysine from degradation relative to other amino acids was also observed.

Amino Acids↗
PubMed1983

Retention time in rumen and degradation of protein supplements fed to lactating dairy cattle.

In part 1 of this experiment, six lactating Holstein cows were used to determine rate of passage of soybean meal, corn gluten meal, and brewer's dried grains through the gastrointestinal tract and also to compare chromium-ethylenediaminetetraacetic acid and cobalt-ethylenediaminetetraacetic acid as liquid digesta markers. Cows were offered a basal diet containing 60% alfalfa hay and 40% of a grain mix twice daily. In addition, all cows received 2.7 kg daily of a mixture of soybean meal, corn gluten meal, and brewer's dried grains. At a designated feeding, each of the protein supplements was labeled with a separate rare earth marker element (samarium, cerium, or lanthanum). Mean ruminal turnover of liquid was 11.4 and 9.9%/h when chromium-ethylenediaminetetraacetic acid and cobalt-ethylenediaminetetraacetic acid were used as markers. Treatment means for ruminal turnover of soybean meal, corn gluten meal, and brewer's dried grains were 4.9, 5.3, and 4.8%/h. In part 2, nitrogen disappearance from soybean meal, corn gluten meal, and brewer's dried grains placed in Dacron bags suspended in the rumen of a fistulated cow was determined at intervals of 1, 4, 8, 12, 16, and 24 h. These measurements, along with information on the amount of protein supplement remaining in the rumen at various intervals (derived from the rate of passage data), were used to estimate protein degradability in the rumen. Protein degradabilities by this approach for soybean meal, brewer's dried grains, and corn gluten meal were 72, 42, and 14%. Results for soybean meal and brewer's dried grains matched in vivo results in the literature, but corn gluten meal was underestimated grossly.

Animals↗
PubMed1983

Influence of insulin and substrate concentration on protein synthetic rate in fetal tissues.

The influence of fetal plasma substrate and insulin concentrations on the fractional synthesis rate or turnover of the mixed proteins in the organs of fetal lambs was calculated from measurements of the rate of uptake of L-14C lysine by the protein in the steady state in utero. The control values for protein synthesis rate in the fetal organs of Welsh mountain ewes, 130 to 140 days' pregnant, were similar to those previously observed in the fetuses of Border Leicester ewes: the mean turnover rates were 38, 72, 40 and 63 per cent per day for the brain, liver, cardiac muscle and the placenta respectively; that for skeletal muscles was 6.8 per cent per day; the corresponding half lives were 2.15, 1.27, 1.8 and 1.42 days respectively and 11.3 days for skeletal muscle. A fivefold increase in the plasma glucose levels had no influence on protein turnover rate over a six hour period. A fourfold increase in the concentration of some amino acids, following a mixed infusion, decreased turnover rate in brain, heart and liver. The half life was increased by 30 per cent, but a small reduction in half life was observed in skeletal muscle. Insulin caused a fall in intracellular lysine concentration in skeletal muscle and in plasma, which suggests that the hormone can reduce muscle protein catabolism in the fetal lamb in the last trimester of gestation. Enhancement of synthesis rate occurred in the presence of the amino acid infusions only; the half life of the proteins was reduced by a factor of three. Insulin also increased placental protein synthesis rate by 50 per cent but had no influence on the other tissues studied.

Amino Acids↗
PubMed1982

Pharmacology of conjugated oestrogens.

Conjugated equine oestrogens (CEE) as in Premarin have been used for the treatment of climacteric complaints and the prevention of oestrogen deficiency symptoms since the early 1940's. The biological activity of Premarin's different components is described. Our knowledge of the pharmacokinetics of the nine structurally related oestrogen conjugates in CEE is reviewed. Particular attention is paid to the significance of the sulphate moiety. It is possible that the sulphate group is exerting a 'protective' effect against sudden oestrogenic stimulation.

Animals↗
PubMed1980

Mapping of cerebral cortical strokes in Rhesus monkeys by laser Doppler spectroscopy.

We have demonstrated the feasibility of using laser doppler spectroscopy to map regional blood flow distribution on the surface of the cerebral cortex. Rhesus monkeys underwent craniotomy, with replacement of part of the calvarium by a transparent plastic dome. A stroke was produced on one side by ligation of the middle cerebral artery. A flow index was computed from a weighted bandwidth of the doppler spectrum of helium-neon laser light scattered from the surface of the cortex, the doppler shift being due to scattering from moving blood cells in the microcirculation. By measuring the doppler flow index at a grid of points over the surface, the region of the stroke was easily delineated, and its progression with time observed. The method provides a rapid measurement with a spatial resolution of about 1 mm.

Animals↗