Raymond R. Paradise

Inhibition by Halothane of Glucose-stimulated Insulin Secretion in Isolated Pieces of Rat Pancreas

In isolated pieces of rat pancreas addition of 300 mg glucose/100 ml bathing medium resulted in a marked and significant increase in insulin secretion. This glucose-stimulated insulin secretion is reversibly inhibited by halothane in a concentration-dependent fashion (0.63, 1.25, and 1.88 MAC halothane produced depressions of 8.4, 18.9, and 37.0 per cent, respectively). Halothane, 1.88 MAC, had no effect on the basal or nonglucose-mediated insulin release.

Increased sensitivity of the isometric contraction of the neonatal isolated rat atria to halothane, isoflurane, and enflurane.

Isolated atria from neonatal (0–5 day old) and adult (50 ± 5 day old) rats were perifused in oxygenated Krebs Henseleit solution at 30 ± 0.5° C and exposed to four different concentrations of halothane, isoflurane, or enflurane while isometric contractile tension was recorded and compared with control atria. ED50 values (mM of anesthetic required to produce 50% reduction in contractile tension) of neonates for halothane (0.18 ± 0.01), isoflurane (0.41 ± 0.05), and enflurane (0.41 ± 0.04) were significantly lower than those of adults (0.35 ± 0.02, 0.80 ± 0.05, and 1.15 ± 0.05, respectively). Furthermore, neonatal ED50 calculated as per cent of adult ED50 was significantly less for enflurane (35%) than for halothane (54%) or isoflurane (51%).

Comparison of cardiovascular effects of thiopental and pentobarbital at equivalent levels of CNS depression.

Thiopental and pentobarbital have been used in high doses to protect the brain from injury following hypoxia or to reduce intracranial pressure. This study was performed to determine whether these barbiturates differ in cardiovascular effects when present in plasma concentrations that produce equivalent CNS effects. The effects of thiopental and pentobarbital on heart rate, stroke volume/kg, cardiac output/ kg, systemic vascular resistance, mean arterial pressure, and central venous pressure were statistically indistinguishable at plasma concentrations of each barbiturate ranging from 50% to 100% of their concentration producing EEG silence. Three of the seven dogs given thiopental developed ventricular bigeminy at plasma concentrations ranging from 45% to 65% of their concentration producing EEG silence. Lidocaine (1.4--2.0 mg/kg intravenously) reversed the bigeminy to sinus rhythm. When given more than the amount needed to produce a flat EEG, five of the seven dogs given thiopental died, but all dogs given pentobarbital survived. Pentobarbital may be a better choice than thiopental when large doses are indicated.

Effect of Morphine on Isolated Human Atrial Muscle

The effects of 5, 10, 20, and 30 mg/100 ml morphine on the force of contraction of isolated human atrial muscle were studied. The tissue was suspended in Krebs-Ringer bicarbonate solution containing 20 mM glucose at 37 C and stimulated with supramaximal voltage at a rate of 70 pulses/min. Morphine, 5 and 10 mg/100 ml, which represents a much higher concentration than would be achieved in plasma following a usual anesthetizing dose of 2 mg/kg, had no effect on force of contraction over a 30-minute period. Twenty and 30 mg/100 ml morphine depressed force of contraction 21 and 70 per cent, respectively. Simulated morphine diluent, containing chlorbutanol and sodium bisulfite alone, had negligible effects on the force of contraction. Pyruvate, 15 mM, had a marked positive inotropic effect in control preparations. The response to pyruvate remained unimpaired in the presence of 5 and 10 mg/100 ml morphine, whereas it was markedly inhibited in the presence of 20 and 30 mg/100 ml morphine. In the 30 mg/100 ml morphine series there was complete recovery of pyruvate response after washout. It is concluded that clinical doses of morphine do not cause myocardial depression. As previously demonstrated with pentobarbital, morphine in excessive doses may exert a negative inotropic effect by interfering with energy production or utilization at a step subsequent to glycolytic formation of pyruvate.

Effects of halothane on glucose-stimulated insulin secretion and glucose oxidation in isolated rat pancreatic islets.

Previous studies have shown that halothane inhibits glucose-stimulated insulin secretion. This study was designed to determine whether the mechanism of inhibition involves a reduction in glucose metabolism. The effects of halothane on glucose (16.7 mM)-stimulated insulin secretion and glucose oxidation were studied in isolated rat pancreatic islets. Halothane, 0.11 mM (0.5 MAC), 0.22 mM (1.0 MAC), and 0.33 mM (1.5 MAC), inhibited glucose-stimulated insulin release in a dose-related manner by 5.2 per cent (NS), 21.0 per cent (P < 0.05), and 32.6 per cent (P < 0.01), respectively. At the 0.33 mM (1.5 MAC) concentration, halothane did not significantly inhibit the oxidation of 6-14C-glucose to 14CO2, although higher concentrations of halothane did result in significant inhibition. The data suggest that halothanes inhibitory effect on glucose-stimulated insulin secretion is not due to interference with glucose oxidation.

Influence of potassium and calcium on behavior of isolated rat atria.

Summary A blotting procedure was developed to remove superficial fluid adhering to rat atrial tissue in a consistent manner prior to chemical analyses. The K+ and water contents of rat atria stimulated for 3 hours in Krebs-Ringer bicarbonate medium containing 6 mM potassium were not significantly different from those of freshly dissected atria, indicating that the medium used in the present investigation is physiological with respect to its potassium concentration and osmotic pressure. Atria stimulated for 2 hours in K+-free medium, after a one hour equilibration period in the normal medium, showed a 57.4% loss in potassium content. No significant change in water content, however, was observed under these conditions. Atria stimulated 200 times per minute in K+-free medium showed idiopathic rhythmicity, contracture, and a temporary increase in contractile force. Self-stimulated atria placed in K+-free medium showed a temporary period of increased rate which was sensitive to the chronotropic action of epinephrine. This was followed by a phase of very slow rate and of epinephrine insensitiveness. Arrhythmias were observed in both phases. The degree of contracture observed was directly related to the rate while the force of contraction was inversely related. The intrinsic rate showed a direct relationship to Ca++ concentration both in presence and absence of potassium.

Multiple Mechanisms of Action of Halothane and Methoxyflurane on Force of Contraction of Isolated Rat Atria

The positive inotropic actions of 5 mM pyruvate and 30 mM fructose were determined in isolated rat atria bathed in normal medium (Krebs-Ringer bicarbonate with 5.5 mM glucose) and in atria depressed by exposure to substrate-free, bicarbonate-free, halothane, methoxyflurane, or low (½ normal) calcium containing medium. Pyruvate produced the maximum increase in force of contraction in atria depressed by substrate-free or bicarbonate-free medium, had less effect (about half the maximum effect) on atria depressed by halothane or methoxyflurane, and had no effect on atria exposed to normal or low-calcium medium. Fructose produced similar results except that it was without effect in atria depressed by bicarbonate-free medium. These results are interpreted as indicating that halothane and methoxyflurane depress the force of contraction by two or more mechanisms: one involving a block in glycolysis as previously demonstrated; another involving another mechanism(s).

The Mechanism of the Positive Chronotropic Action of Diethyl Ether on Rat Atria

Diethyl ether elicited a dose-dependent increase in the intrinsic frequency of contraction of isolated rat atrial preparations. The maximum effect (+34 per cent) occurred with 230 mg ether/100 ml medium. This ether concentration corresponds to a partial pressure of 29.2 mm Hg or 2 MAC. The positive chronotropic action of ether was not reduced in atria obtained from rats pretreated with reserpine (4 mg/kg, ip) although this treatment markedly reduced the effect of tyramine on frequency of contraction. The positive chronotropic response to 0.01 µM isoproterenol was inhibited by the beta-adrenergic antagonist 0.3 µM dl-propranolol but remained unimpaired in the presence of 0.3 µM d-propranolol (a much weaker antagonist). In contrast, the atrial response to ether was similar in the presence of either d- or dl-propranolol. Atropine, in concentrations that completely blocked the negative chronotropic action of acetylcholine, did not increase the frequency of contraction, suggesting that the positive chronotropic effect of ether is not due to an atropine-like activity of ether. Our results indicate that the positive chronotropic effect of ether on isolated rat atrial preparations is not mediated via catecholamine release, nor does it represent direct stimulation of beta-adrenergic receptors or block of cholinergic receptors.

Effect of Prolonged Starvation on the Functional Status of the Isolated Rat Atria

Summary Starvation results in a marked loss of body weight in rats, e.g., a 30% loss in 4 days after which the animals begin to die. By the sixth day only 5 of the 20 were still living. Despite these drastic effects on the rat as a whole, cardiac function, as determined by developed tension of isolated atria in glucose-containing medium, was as good or better than that from fed rats. When placed in substrate-free medium, atria from fed rats showed a marked decline in developed tension, e.g., a 44% decline in 40 min. In contrast, the developed tension of atria from rats starved 1 to 6 days showed a smaller rate of decline with the least decline seen in atria from rats starved for 2 days. These results suggest (a) that prolonged starvation has no deleterious effect on cardiac function and (b) starvation increases the storage of readily metabolizable substrate useful for the functional activity of the atria.

The Mechanism of the Negative Inotropic Effect of Methoxyflurane on Isolated Rat Atria

The force of contraction of isolated rat atria suspended in Krebs-Ringer bicarbonate medium containing 5.5 mM glucose was depressed 55 per cent by approximately 9 mg/100 ml of methoxyflurane in the medium bathing the atria. This corresponds to a partial pressure of 1.84 torr (0.24 vol per cent). Addition of fructose (30 mM) or pyruvate (5 mM) to atria depressed by methoxyflurane resulted in a marked positive inotropic action. Additional glucose (20 mM) was relatively ineffective in atria similarly treated. These results suggest an interference with glucose utilization at an early step in glycolysis above the phosphofructokinase step. Methoxyflurane had its usual depressant action in the absence of exogenous glucose (substrate-free medium). Thus, glucose uptake and phosphorylation were ruled out as the site of action of methoxyflurane. These results suggest that methoxyfluranes depressant action results from a block at the glucose phosphate isomerase step, the step responsible for converting glucose-6-phosphate to fructose-6-phosphate. Multiple sites of action are not ruled out by these results.