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D R Hunter

Publications and source records attributed to D R Hunter.

At least 37 records · Page 2Linked to original sources
PubMed1982

Cellular calcium turnover in the perfused rat heart: modulation by caffeine and procaine.

Pool A is a rapidly exchangeable cellular pool of Ca++ whose release is triggered by a mechanism involving extracellular Ca++ (Hunter et al., 1981). We have now found that pool A is rapidly released from the perfused rat heart when 10 mm caffeine is added to the perfusate. Pool A release by caffeine was demonstrated during a Ca++ free perfusion. When the perfusate contained 2.5 mm Ca++, caffeine induced an immediate contractile failure. The steady state level of pool A (normally 69 +/- 10 nmol Ca++/g wet wt heart) was also decreased by 60%. Pool A was similarly depleted in hearts perfused with medium containing 0.2 mm Ca++. Procaine (5 mm) inhibited by 50% the release of pool A triggered by caffeine and inhibited by 86% the release triggered by extracellular Ca++. The inhibition of Ca++-induced release of pool A by procaine was partially relieved by externally stimulating the hearts. External stimulation also decreased the inhibition by procaine of Ca++ uptake by pool A from 83% to 28%. These results are further evidence that pool A is located in the sarcoplasmic reticulum and that release of pool A to the myofibriles is triggered by excitation-dependent Ca++ influx.

Action Potentials↗
PubMed1981

Measurement of rapidly exchangeable cellular calcium in the perfused beating rat heart.

Although Ca2+ has long been known to play a vital role in excitation--contraction coupling in the heart, investigation of the details of this role has been hampered by the experimental difficulty of measuring Ca2+ movements through the different compartments of the cell. A major problem has been to distinguish the relatively small amount of rapidly exchangeable cellular Ca2+ from the large amount of vascular and interstitial Ca2+. We report here a method that overcomes this problem. Rat hearts were labeled by perfusion at 37 degrees C with medium containing 45Ca2+. They were then cooled, and extracellular 45Ca2+ was removed by perfusion at 6 degrees C with Ca2+-free medium. Cellular 45Ca2+ that had been trapped in the hearts by cooling was then released by reperfusion at 37 degrees C with medium containing unlabeled Ca2+. The cellular origin of this 45Ca2+ was confirmed by using [3H]sucrose: When hearts were also labeled with [3H]sucrose, very little [3H]-sucrose was released with the 45Ca2+ peak. The amount of exchangeable cellular Ca2+ in hearts labeled for 5 min was 125 +/- 13 nmol/g of wet weight. The half-time for its release was less than 1 min. This cellular Ca2+ contained at least two pools: a rapidly exchanging pool that required extracellular Ca2+ for release (pool A, 38% of total), and a more slowly exchanging pool that did not (pool B, 62% of total). Hearts stimulated with isoproterenol during the 5-min labeling period showed an increase of 46% for the total amount of exchangeable cellular Ca2+; this increase was entirely located in pool A.

Animals↗
PubMed1981

Contracture in isolated adult rat heart cells. Role of Ca2+, ATP, and compartmentation.

Isolated intact quiescent myocytes from the adult rat were used as a model system for investigating the determinants of contracture induced by metabolic deprivation. The model simulated the pattern of contracture and ATP decline seen in the intact heart during ischemia. Three new insights into the contracture process were gained: (1) in the quiescent cell system, the rate of onset of contracture was independent of external Ca2+, supporting the view that the Ca2+ dependence of the rate of onset in the whole heart is related to beat-dependent substrate utilization; (2) the second phase of ATP decline was paralleled by a decline in the percentage of cells which had not undergone contracture, suggesting that-in any cell-contracture is immediately preceded by a total loss of ATP; and (3) oligomycin delayed the onset of contracture by 55 +/- 12%, suggesting that mitochondrial ATPase activity is a significant drain on energy resources in the quiescent ischemic heart.

Adenosine Triphosphate↗
PubMed1980

Allosteric inhibition of the Ca2+-activated hydrophilic channel of the mitochondrial inner membrane by nucleotides.

The control by nucleotides of the Ca2+ -activated channel which regulates the nonspecific permeability of the mitochondrial inner membrane has been investigated quantitatively. The cooperative binding of two molecules of ADP to the internal (matrix) side of the channel causes a mixed-type inhibition of channel activity. ATP, AMP, cAMP and GDP are all ineffective. NADH shows a pattern of inhibition similar to that of ADP, though the apparent KI is higher by a factor of 200. NADPH relieves the inhibition by NADH. NAD+ also inhibits, b,t its affinity is a factor of 10 less than that of NADH. When ADP and NADH are added together, they act synergistically to inhibit the Ca2+-activated channel. It is concluded that the concept of the modification of enzyme activity by the allosteric binding of nucleotides, which is well established for soluble enzyme systems, also has application to the regulation of channels that control membrane permeability.

Adenine Nucleotides↗
PubMed1980

Comparison of Ca2+, Sr2+, and Mn2+ fluxes in mitochondria of the perfused rat heart.

The amount of readily exchangeable Ca2+ in mitochondria of an isolated working rat heart is less than 10 ng-ions/g heart. We therefore conclude that either no Ca2+ enters mitochondria or that the Ca+ which does enter is removed continuously. Using Sr2+ and Mn2+, we obtained evidence that the mitochondrial Na+-Ca2+ exchanger was indeed operational in releasing metal from mitochondria of the heart. When Ca2+ in the perfusate was replaced by Sr2+, we found that a significant amount of Sr2+ (approximately 100 ng-ions/g heart) entered mitochondria. When the heart then was returned to a Ca2+-containing perfusate, over 80% of the Sr2+ was washed out of mitochondria within 30 seconds. When low levels of Mn2+ were added to the perfusate, we found that Mn2+ accumulated in mitochondria irreversibly. This is evidence for the operation of the Na+-Ca2+ exchanger because Na+ was found to release Ca2+ and Sr2+ but not Mn2+ from isolated rat heart mitochondria. Our estimates indicate that when the Na+-Ca2+ exchanger is maximally operative, as in the Sr2+-perfused heart, the flux of Sr2+ through mitochondria is at most 10% of the total flux needed for the activation of contraction. The low level of Ca2+ in the mitochondria of Ca2+-perfused hearts suggests a much smaller flux of Ca2+ through the mitochondria in this case. We therefore conclude that mitochondria play little if any role in the beat-to-beat regulation of normal Ca2+ fluxes in the rat heart.

Animals↗
PubMed1976

Relationship between configuration, function, and permeability in calcium-treated mitochondria.

Low levels of calcium (100 nmol/mg) added to beef heart mitochondria induced a configurational transition from the aggregated to the orthodox state and a simultaneous uncoupling of oxidative phosphorylation. The primary effect of calcium was to cause a nonspecific increase in the permeability of the inner membrane, resulting in entry of sucrose into the matrix space and the observed configurational transition. The uncoupling and permeability change induced by calcium could readily be reversed by lowering the calcium:magnesium ratio in the presence of either substrate or ATP. The configurational state, however, remained orthodox. This, along with studies of hypotonically induced orthodox mitochondria in which the membrane remained coupled and impermeable until after the addition of calcium, led to the conclusion that coupling was related to the permeability state of the inner membrane rather than the configurational state. Phosphate, arsenate, or oleic acid was found to cause a transition similar to that induced by calcium. Studies with the specific calcium transport inhibitors, EGTA, ruthenium red, and lanthanum revealed that endogenous calcium is required for the anion-induced transitions. A single mechanism was further indicated by a common sensitivity to N-ethylmaleimide. Strontium was ineffective as an inducer of the transition, even though it is transported by the same mechanism as calcium. This indicates that there are additional calcium-binding sites responsible for triggering the transition. Magnesium and calcium appeared to compete for these additional sites, since magnesium competitively inhibited the calcium-induced transition, but had no effect on calcium uptake. Calcium was found to potently inhibit the respiration of all NAD+-requiring substrates prior to the transition. Strontium also produced this inhibition without a subsequent transition. ATPase activity was induced at the exact time of transition with calcium and was not induced by strontium. This suggests that calcium-induced ATPase uniquely required the transition for activity, in contrast to the ATPase induced by uncoupler or valinomycin. The results of this work indicate that mitochondria have a built-in mechanism which responds to low levels of calcium, phosphate, and fatty acids, resulting in simultaneous changes, including increased permeability, inducation of ATPase, uncoupling of oxidative phosphorylation, and loss of respiratory control.

Adenosine Triphosphatases↗
PubMed1975

Temporal bone radiography: a screening survey approach.

For preliminary evaluation of the temporal bone, a combination of conventional radiographs and hypocycloidal tomography in the frontal projection is advocated. If significant abnormalities are detected in this screening survey, a more in-depth study can then be pursued.

Humans↗