Search

The rubidium efflux from hypothermic rat hearts perfused by the Langendorff method at 20 degreesC was studied. At this temperature 87Rb-NMR efflux experiments showed the existence of two 87Rb pools: cytoplasmic and mitochondrial. Rat heart mitochondria showed a very slow exchange of mitochondrial Rb+ for cytoplasmic K+. After washout of cytosolic Rb+, mitochondria kept a stable Rb+ level for >30 min. Rb+ efflux from mitochondria was stimulated with 0.1 mM 2, 4-dinitrophenol (DNP), by sarcolemmal permeabilization and concomitant cellular energy depletion by saponin (0.01 mg/ml for 4 min) in the presence of a perfusate mimicking intracellular conditions, or by ATP-sensitive K (KATP) channel openers. DNP, a mitochondrial uncoupler, caused the onset of mitochondrial Rb+ exchange; however, the washout was not complete (80 vs. 56% in control). Energy deprivation by saponin, which permeabilizes the sarcolemma, resulted in a rapid and complete Rb+ efflux. The mitochondrial Rb+ efflux rate constant (k) decreased in the presence of glibenclamide, a KATP channel inhibitor (5 microM; k = 0.204 +/- 0.065 min-1; n = 8), or in the presence of ATP plus phosphocreatine (1.0 and 5.0 mM, respectively; k = 0.134 +/- 0.021 min-1; n = 4) in the saponin experiments (saponin only; k = 0.321 +/- 0.079 min-1; n = 3), indicating the inhibition of mitochondrial KATP channels. Thus hypothermia in combination with 87Rb-NMR allowed the probing of the mitochondrial K+ pool in whole hearts without mitochondrial isolation.

Richardson's ground squirrels [body temperature (Tb) 7 degrees C] survive prolonged stable hypothermia for three times as long as do rats (Tb 19 degrees C) (72 vs. 24 h). We have examined the changes in blood gases and acid-base state to assess whether these contribute to this difference in survival time. None of the variables (measured at ambient temperature of 25 degrees C) differed significantly between rats and ground squirrels before hypothermic induction. During cooling, neither hematocrit nor plasma lactate changed significantly, but arterial and venous PO2 and PCO2 increased and arterial and venous pH decreased in both groups. During prolonged hypothermia, hematocrit increased significantly in rats (58.8 +/- 1.7% at 24 h) but not in ground squirrels (39.1 +/- 1.0% at 72 h). Both species maintained stable arterial blood gases but showed decreased venous PO2; arterial and venous pH decreased significantly with time in both species in conjunction with increased plasma lactate. These patterns of decreased venous PO2 and increased plasma lactate suggest that reduced tissue oxygenation occurs during hypothermia. This happens earlier in rats at a Tb of 19 degrees C than in ground squirrels at a Tb of 7 degrees C, possibly as a result of increased hematocrit in hypothermic rats. Remedial measures directed at improving tissue O2 delivery may therefore prolong the hypothermic survival of rats.

We investigated the contribution of maximal Ca(2+)-activated force to the positive inotropism induced by mild hypothermia. Phosphorus-31 nuclear magnetic resonance spectroscopy revealed that neither energy-related phosphorus compounds in myocardium nor intracellular pH was responsible for the change in contractility. Maximal Ca(2+)-activated pressure (MCAP), the intact-heart correlate of maximal Ca(2+)-activated force, was determined in isolated perfused rabbit hearts by measuring isovolumic left ventricular pressure during tetani at extracellular Ca2+ concentrations greater than or equal to 10 mM. Tetani were elicited by rapid pacing after exposure to ryanodine. MCAP increased by 2.17 +/- 0.28% (mean +/- SE, P less than 0.001, n = 19) for each degree of myocardial cooling between 30 and 38 degrees C. Our results indicate that a primary change in myofilament Ca2+ responsiveness underlies the positive inotropism in hypothermia. The increase in maximal Ca(2+)-activated force may explain the observation of positive inotropism without an upward shift in the relation between oxygen consumption and pressure-volume area, as previously reported for cooled whole hearts.

The regulation of tissue pH during hypothermia is important for cellular homeostasis. The present study was undertaken to determine the relationship between blood pH and intracellular pH in the brain and heart during hypothermia in sheep and to compare these data with those in humans. Intracellular pH (pHi) was determined by 31P nuclear magnetic resonance (NMR) spectroscopic data collected from the heart and brain of sheep during cardiopulmonary bypass (CPB). Alpha-stat and pH-stat blood pH management schemes were compared. When the blood pH was held constant (pH stat), the pHi of the heart increased from 7.01 +/- 0.01 at 37 degrees C to 7.18 +/- 0.02 at 26 degrees C, and the pHi of the brain increased from 7.04 +/- 0.02 at 37 degrees C to 7.23 +/- 0.02 at 26 degrees C and to 7.32 +/- 0.04 at 20 degrees C. Alpha-stat pH management resulted in similar increases in pH to that found with pH-stat [heart: 7.00 +/- 0.02 at 37 degrees C to 7.19 +/- 0.03 at 26 degrees C; brain: 7.07 +/- 0.02 at 37 degrees C to 7.29 +/- 0.02 at 26 degrees C, and 7.32 +/- 0.03 at 20 degrees C]. The tissue pH of the heart in humans showed similar findings during blood pH-stat regulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Impairment of metabolic substrate mobilization and utilization may be a factor limiting survival in hypothermia. Using a newly developed technique for maintaining stable low body temperature (Tb), substrate profiles and their regulation by glucagon were examined in hypothermic rats (Tb 19 +/- 0.3 degrees C) over 20 h. During cooling, plasma glucagon, glucose, and free fatty acid (FFA) concentrations increased significantly (536 +/- 55 pg/ml, 304 +/- 26 mg/100 ml, and 844 +/- 81 mueq/l, respectively). Plasma glucagon and glucose concentrations continued to increase up to 8 h (peaks 810 +/- 103 pg/ml and 451 +/- 33 mg/100 ml, respectively) and remained high throughout the rest of the hypothermic period. FFA concentrations decreased steadily during the hypothermic period. Exogenous glucagon (20 micrograms/kg) induced significant increases in plasma glucose (+129 +/- 31 mg/100 ml) and FFA concentrations (+351 mueq/l) at 2 h but had no effect at 15 h of hypothermia. In vitro evaluation of pancreatic alpha-cell function indicated that glucagon secretion is independent of temperature between 37 and 19 degrees C. Our data indicate that hypothermia is characterized by a disturbed substrate metabolism, which is likely due to an imbalance in pancreatic alpha- and beta-cell function and a time-dependent decrease in tissue sensitivity to glucagon. These deleterious changes may limit survival in hypothermia.

Previous studies have shown that organic bases, including some drugs, are secreted by renal proximal tubules. The present studies examined the transport of the organic bases cimetidine and procainamide by rabbit proximal straight tubules perfused in vitro. Both drugs were secreted into the tubule lumen. [3H]cimetidine secretion was reduced by quinidine, procainamide, and N-acetylprocainamide. Previous studies showed that cimetidine secretion was reduced by other organic bases. Hypothermia and ouabain inhibited [3H]procainamide secretion as was shown previously for cimetidine secretion. [3H]procainamide secretion was also reduced by quinidine, cimetidine, procainamide, and N-acetylprocainamide but not by probenecid. High concentrations of cimetidine (10(-3) M) had no effect on the rates of fluid or total CO2 absorption. When analyzed in terms of Michaelis-Menten kinetics, the effect of cimetidine on procainamide secretion and procainamide on cimetidine secretion was consistent with competitive inhibition. The results suggest that both cimetidine and procainamide are secreted into the lumen of proximal straight tubules predominately by an organic base transport mechanism. These studies raise the possibility that some of these drugs might compete for a common secretory mechanism in renal tubules and reduce the elimination of each other.

Among the biochemical processes initiated by anoxia or ischemia that play a central role in cellular injury during deprivation is an alteration in cellular hydrogen ion concentration. In this study, the rate of exchange of intracellular hydrogen ion concentration in canine brain was compared with that in liver, using the 5,5-dimethyloxazolidine-2,4-dione method during anoxia at 37 and at 20 degrees C. The intracellular pH of brain decreased more rapidly than it did in liver anoxia at 37 degrees C. The intracellular pH of neither brain nor liver changed substantially during 30 min of anoxia at 20 degrees C. Although the ratio of tissue to plasma water did not change, the calculated extracellular-to-intracellular volume ratio increased during 30 min of anoxia at 37 but not at 20 degrees C.

Hemodynamic functions and blood viscosity changes in hypothermia (core approximately 25 degrees C) were studied in 14 pentobarbital-anesthetized dogs subjected to surface cooling. The viscosity of blood (eta B) increased progressively to 173% of that at 37 degrees C when body temperature was lowered to 25 degrees C. The increase in blood viscosity was caused by: a) the direct effect of low temperature on plasma viscosity, b) hemoconcentration as a result of plasma loss, and c) the low-flow (low-shear) state induced by hypothermia. A larger portion of the increased viscosity was caused by the low-flow state in hypothermia. The systemic flow resistance (SFR) increased to 271% of control, and this was attributable about equally to the increases in blood viscosity and systemic vascular hindrance (SFR/eta B). Similarly, the viscosity of blood contributed significantly to raising the pulmonary flow resistance. The relative constancy of mixed venous O2 saturation suggests that the cardiac output at low body temperature is generally adequate to meet the metabolic needs.