Quarterly journal of experimental physiology (Cambridge, England)最新文献

The dependence of adenosine release on blood flow was investigated in greyhounds anaesthesized with sodium pentobarbitone and artificially ventilated. The gracilis muscles were neurally and vascularly isolated, and perfused at constant flow rates of 42% (low), 89% (medium) or 132% (high) of their maximum free flow during contraction induced by stimulation of the motor nerve. Stimulation produced contractions whose force declined from 716 +/- 60 to 464 +/- 46 g (100 g)-1 over 10 min. Resting perfusion pressure increased in line with the flow rate, but the fall in resistance accompanying contractions varied reciprocally with the flow (57 +/- 2.9, 39.6 +/- 6.6 and 15.3 +/- 5.6% at low, medium and high flows respectively). Venous PO2 decreased during contraction to 26.6 +/- 6.2 mmHg at 'low', 31.5 +/- 5.1 mmHg at 'medium' and 37.2 +/- 1.7 mmHg at 'high' flows. Venous plasma adenosine concentration increased significantly above resting levels during contraction at all flow rates. Adenosine release at low flow (12.0 +/- 2.7 nmol min-1 (100 g)-1) was significantly greater than that at medium or high flows (5.6 +/- 1.3 and 4.1 +/- 1.3 nmol min-1 (100 g)-1 respectively), but the latter were not different from each other. There was no correlation between adenosine release and either venous oxygen tension during muscle contraction or the ratio of oxygen supply to free-flow oxygen consumption. These data suggest that the mechanism underlying adenosine release during muscle contraction may be independent of oxygen lack.

Stretch of active whole muscle causes a sudden yielding of the sarcomeres when the actin and myosin filaments are displaced by 10-12 nm from their isometric position. This behaviour is not normally seen during stretch of singly-dissected fibres, but it can be induced by introducing series compliance into the system.

Possible mechanisms for calcium entry at positive membrane potentials were investigated in single cells isolated from guinea-pig ventricular muscle. The cells were voltage clamped and contraction was measured by an optical technique. When prolonged (200 ms to 2 s) depolarizations at +60 mV were applied, contraction amplitude increased with pulse duration, in contrast to the contraction at 0 mV. When a 'pre-pulse' to 0 mV was applied for 200 ms to inactivate current through 'L-type' calcium channels, contraction nevertheless increased with membrane potential during a subsequent test pulse applied over the range -40 to +60 mV. Contraction during the test pulse at +60 mV was abolished when extracellular calcium was reduced to zero. This effect developed more rapidly than abolition of the contraction in response to the pre-pulse to 0 mV. Reduction of extracellular calcium from 2.5 to 1 mM reduced the contraction at +60 mV to a greater extent than that at 0 mV and caused an inward shift in the current at +60 mV. Nifedipine (5 microM) substantially reduced the contraction during the test pulse to 0 mV but had little effect on the contraction at +60 mV. Conversely, dodecylamine (20 microM) caused little or no decrease in the contraction at 0 mV but substantially reduced the contraction at +60 mV. Following a conditioning pre-pulse to 0 mV the contraction at +60 mV was not consistently reduced by exposure to 3 microM-ryanodine. The interpolation of a single 200 ms pulse to +60 mV in a train of pulses to 0 mV potentiated the following contraction to 0 mV. This potentiation decayed over the first four steps to 0 mV following an interpolated pulse and increased with the voltage of the interpolated pulse over the range -20 to +60 mV. Potentiation was abolished on exposure to 3 microM-ryanodine. These observations are consistent with entry of calcium at positive membrane potentials through voltage-dependent, non-inactivating pathways which are insensitive to nifedipine but inhibited by dodecylamine. The observations support the hypothesis that calcium entry via this mechanism may contribute, at least under some conditions, to the loading of intracellular stores of calcium during the late plateau of the action potential, and thus influence subsequent contraction. Calcium entry through Na+-Ca2+ exchange is a possibility which would allow calcium entry to increase over the range of membrane potentials at which contraction was increased. However, additional calcium entry through other nifedipine-insensitive pathways, such as calcium-activated non-selective channels, cannot be excluded.

Experiments are described showing unequivocally that transmission at the squid giant synapse can be reversibly blocked by L-glutamate and its agonists kainate, quisqualate and AMPA, though not by NMDA. This effect is presumably brought about by desensitization. The glutamate antagonists cis-2,3-PDA, GAMS and the new quinoxalinediones CNQX and DNQX are also potent reversible blockers. These findings provide new evidence that L-glutamate is a transmitter at the giant synapse and further suggest that the glutamate receptor may be of the non-NMDA type.

The responsiveness of the fetal adrenal to a rise in either endogenous or exogenous ACTH (adrenocorticotrophic hormone) was examined acutely in piglets between 70 and 105 days gestation (term, 115 days). In addition the pre-partum changes in plasma ACTH and cortisol were followed in chronically catheterized fetuses and the effect of a continuous intrafetal ACTH infusion on the time of delivery was also investigated. The acute experiments on ten sows were carried out (under sodium pentobarbitone) on twenty-five fetuses, sampled from a branch of the umbilical artery with minimal disturbance. A second sample was taken 10-20 min later after exteriorization or surgery (catheterization). Such fetal manipulation resulted in significant increases in plasma ACTH and cortisol from 70 days gestation and the responses increased with fetal age. No corresponding changes were seen in the controls or in the sow over the same period. An exogenous bolus of ACTH (200 ng ml-1 I.V.) evoked rises in fetal plasma cortisol comparable with the endogenous changes (+11 +/- 2 ng ml-1, n = 4). Chronic experiments were carried out on fourteen sows catheterized under sodium pentobarbitone anaesthesia at 95-100 days gestation; in fetuses sampled until delivery (at 111-114 days) gradual rises in both fetal plasma cortisol and ACTH were observed. There was a highly significant positive correlation between plasma cortisol and log plasma ACTH (r = 0.81, n = 52, P less than 0.001). Analysis of the basal values from the 70-100 day fetuses also showed a positive correlation (r = 0.47, n = 23, P less than 0.05) but the slope of the regression line was significantly less than that for the older fetuses indicating a greater adrenocortical response to a given level of ACTH nearer term. In five sows a continuous infusion of ACTH (0.125 mg day-1 for 4-5 days from about 100 days) was given to one or two fetuses per litter (total size, 7-12). This treatment resulted in a rise in fetal plasma cortisol to 85.4 +/- 8.9 ng ml-1, which was equivalent to that found during labour, but did not induce premature parturition.

We have investigated the role of gastrin in the development of the gastrointestinal tract during the latter part of gestation in fetal sheep. We surgically removed the major source of gastrin, the gastric (abomasal) antrum, from five fetuses at 90 days of gestation. The remaining abomasum was anastomosed to the pylorus allowing unobstructed flow of luminal contents. Another five fetuses, subjected to sham-antrectomies at 90 days, served as controls. Further surgery was performed in all fetuses at 120 days for the placement of vascular catheters to permit measurement of plasma gastrin concentrations. The fetuses were infused with [3H]thymidine to study villus cell migration rates. At 135 days of gestation samples of gastric (abomasal) fundus, and proximal and distal small intestine, were processed for histology and morphometric analysis. The antrectomized fetuses had significantly lowered plasma gastrin concentrations (P less than 0.025) between 120 and 135 days. At 135 days, the mean body weight, crown-rump length, total gut weight and small intestinal weight and length were not significantly different between the two groups. Similarly, there were no significant differences between groups in the mean thicknesses of the gut wall, mucosa and muscularis externa, or in the mean villus height and crypt depth in the proximal or distal parts of the small intestine. Villus cell migration rate in the proximal and distal small intestine was not affected by antrectomy. No simple relationship could be demonstrated between any of these parameters and plasma gastrin concentration. In the antrectomized fetuses, the mean crypt density and crypt-to-villus ratio were significantly reduced in the proximal small intestine (P less than 0.05), while only the density of villi was reduced in the distal small intestine (P less than 0.05). In the antrectomized fetuses there were significant correlations between plasma gastrin and the fraction of fundic mucosa occupied by gland and pit (P less than 0.005), and between plasma gastrin concentration and villus density (P less than 0.01) and crypt-to-villus ratio (P less than 0.025) in the proximal small intestine. In the sham group these correlations were absent. We conclude that the removal of the gastric antrum in fetal sheep results in decreased plasma gastrin concentration, and that gastrin appears to have a regulatory or trophic role on the gut mucosa in these circumstances.(ABSTRACT TRUNCATED AT 400 WORDS)

A microangiographic technique, using novel methods of contact X-ray microscopy, was developed and used to study the effects of ionic (Urografin 370) and non-ionic (Omnipaque 350) radiographic contrast media, and hyperkalaemia (exposure to 15, 30 and 60 mM-K+ solutions) on coronary artery calibre and total coronary vascular resistance in isolated, Langendorff-perfused rabbit hearts. Repeated injection of both types of contrast media produced similar haemodynamic effects and some reduction in coronary diameter, but greater variation in arterial calibre was observed with Urografin (P less than 0.001). Perfusion with hyperkalaemic solutions produced a dose-related increase in total coronary vascular resistance (P less than 0.001), but this was associated with a complex pattern of changes in large-vessel calibre. The direction and magnitude of the responses to hyperkalaemia varied depending on the site within the artery studied; in general hyperkalaemia produced a dose-related vasoconstriction which was greater distally than proximally. Thus, with 15 mM-K+ vasoconstriction occurred in the left anterior descending artery (4.3% proximally; 15.6% distally) which increased to 40.7 and 52.1% (proximally and distally, respectively) with 30 mM-K+. With 60 mM-K+ further diffuse and segmental vasoconstriction occurred in 4/5 and 1/5 hearts respectively (mean values 31.5 and 56.7% at the proximal and distal sites). During perfusion with 15 mM-K+, net vasodilatation (17% at both the proximal and distal sites) was noted in the left ventricular branch of the circumflex artery, but with 30 mM-K+ vasoconstriction (18.3% proximally and 38.0% distally) occurred. Thus, while hyperkalaemia consistently increased coronary vascular resistance, these changes were associated with marked variation in the extent and distribution of in situ constriction and dilatation within the coronary circulation.

The effects of Ca2+-active agents on ovine salivary flow rate and composition were measured in parotid glands under several conditions: no stimulation, submaximal stimulation of the secretomotor nerve, or stimulation by the cholinergic agents carbachol or bethanechol. Agents were infused into the arterial blood supply of parotid glands and those investigated were: calcium chloride, the calcium ionophores Bay K8644 and A23187, the calcium chelators EGTA and EDTA, the voltage-dependent calcium channel blocker verapamil and congeners, the calmodulin inhibitor trifluoperazine (TFP) and congeners. None of the agents affected the flow rate of saliva from unstimulated or pharmacologically stimulated glands. Increased plasma [Ca2+] and the ionophores did not affect salivary flow in nerve-stimulated glands. In nerve-stimulated glands, EGTA and TFP reduced salivary flow rate and verapamil increased it. The effect of EGTA was reversed by restoring plasma [Ca2+] to normal (1.0-1.2 mmol/l) or above, but the responses to TFP and verapamil were not reversed by increasing plasma [Ca2+]. In all three conditions of stimulation, infusions of EGTA, verapamil or TFP increased salivary [HPO4(2-)] and reduced [HCO3-] and pH. The ionophores had the opposite effects but increased plasma [Ca2+] had no effect. At the same time, EGTA, verapamil or TFP increased salivary [Na+ + K+], Bay K8644 had the opposite effect but increased plasma [Ca2+] had no effect. The osmolality of the saliva was not altered in any of these circumstances. Salivary [Ca2+] was increased by Ca2+ infusion and reduced by EGTA. Glandular blood flow increased with infusion of agents which increased salivary [HPO4(2-)], fell with infusion of ionophores, and was unchanged by increased plasma [Ca2+]. Thus, there appear to be three calcium-related activities in ovine parotid salivary gland in vivo: (1) salivary flow rate by action at the neuroeffector site, (2) salivary composition by alteration of the ratio of HPO4(2-): HCO3-, and (3) rate of blood flow through the gland by altering vascular resistance.

Actions of halothane were investigated under voltage-clamp conditions in single cells from guinea-pig ventricular muscle. Contraction (measured by an optical method) evoked by step depolarization to 0 mV was consistently reduced by halothane. At positive membrane potentials (+60 mV) 2% halothane did not cause a consistent depression of peak contraction, and in the majority of cells contraction was enhanced. Two per cent halothane increased the time-to-peak contraction at +60 mV. However, when a pre-pulse to 0 mV was applied to inactive calcium current through L-channels, any effect of 2% halothane on the time-to-peak of contraction was reduced or abolished. A halothane-induced increase in time-to-peak contraction was also observed at membrane potentials in the range of the action potential plateau (+20 and +40 mV). In double-pulse experiments contraction during a 'test' depolarization to +60 was measured following a 'conditioning' depolarization to 0 mV. Contraction at +60 mV was slightly reduced at brief interpulse intervals (less than 400 ms) following the 'conditioning' depolarization to 0 mV, and recovered as the interval was prolonged; in cells exposed to halothane contraction at +60 mV was no longer influenced by the interval between the pulses. Isoflurane (3.2%) had qualitatively similar but less potent effects than halothane on contraction at +60 mV. These observations are consistent with the suggestion that mechanisms for calcium entry may vary with the membrane potential: at 0 mV, the major pathway for calcium entry may be through halothane-sensitive L-type calcium channels, while at +60 mV entry may be through additional pathways which are relatively resistant to halothane. Actions of halothane on the time-to-peak of contraction may be accounted for by its influence on the sarcoplasmic reticulum to decrease net uptake and release of calcium. These actions of halothane might be of importance during the action potential plateau.

Factors determining the digestive efficiency of donkeys were studied in animals fed either a low quality roughage (wheat straw: 77.1% neutral detergent fibre, 2.8% crude protein) or a high quality forage (alfalfa hay: 47.5% neutral detergent fibre, 22.7% crude protein). The neutral detergent fibre (NDF) intake when fed wheat straw was 1693 +/- 268 g animal-1 day-1, 10% higher than when fed alfalfa hay. Digestive coefficient of NDF and acid detergent fibre (ADF) when fed wheat straw amounted to 50.9 +/- 4.9 and 42.0 +/- 4.1% respectively. NDF and ADF apparent digestibilities and mean retention times (37.7 +/- 1.7 and 36.4 +/- 3.2 h respectively) were not significantly different (P greater than 0.05) between the two diets. The donkey appears to digest cell wall constituents as efficiently as the Bedouin goat when on low quality roughage, but less efficiently when fed alfalfa hay. Its energy digestibility is, however, as high as that reported for the Bedouin goat. The donkey's high energy digestibility is related to its capacity to digest soluble food components more efficiently than the ruminant. The mean retention time in the donkey is shorter than in the Bedouin goat and is consistent with its capacity to compensate for a lower quality diet by increasing its intake rate. Recycling of urea in donkeys maintained on wheat straw amounted to 75.5 +/- 13.0% of the entry rate. A decrease in the rate of renal urea filtration, coupled with an increase in the fraction reabsorbed, increased the retention of nitrogenous waste and permitted recycling of nitrogen into the gut.