P.A. Poole-Wilson

Hypoxia and calcium

Abstract During hypoxia peak developed tension falls rapidly, resting tension inceeases slowly and recovery is incomplete if the hypoxia is sufficiently prolonged for resting tension to become greatly elevated. We propose that the initial fall of peak developed tension is due to a failure of excitation-contraction coupling at a time when high energy phosphate stores are only partially depleted. The rise of resting tension appears to reflect a deficiency of ATP necessary for the maintenance of intracellular calcium homeostasis. The removal of extracellular calcium, the addition of verapamil, and the measurement of calcium influx, indicate that the initial rise in resting tension is not due to an inward movement of calcium from the extracellular phase. Mechanical recovery after a period of hypoxia is jeopardized when there has been a substantial net gain of calcium.

Contractile responses of isolated adult rat and rabbit cardiac myocytes to isoproterenol and calcium.

Myocytes were isolated by Langendorff perfusion of rat or rabbit hearts with low calcium solution followed by collagenase and hyaluronidase, or by incubation of chunks of rat ventricular tissue in similar media. Cells were then placed in a bath on a microscope stage, superfused and electrically stimulated. Contraction amplitude and rate of change of length during contraction were measured using a video camera and edge detection monitor. Cells were selected for study using a number of criteria developed to identify and define a cell population able to give consistent inotropic responses over a long period. The maximum contraction amplitude with isoproterenol in rabbit cells was 0.244 micron (sarcomere length change) or 13.1% (percentage change in cell length), and the EC50 was 12.8 nM. The maximum contraction amplitude with isoproterenol did not differ significantly between rat and rabbit, between cells prepared by perfusion and those made from chunks, or when determined from non-cumulative rather than cumulative curves. The EC50 for isoproterenol in rat cells made by the perfusion method (cumulative curves) was 3.81 nM, significantly lower than in rabbit. The maximum amplitude obtained with increasing concentrations of calcium was not significantly different from that with isoproterenol under any condition. The EC50 for calcium averaged 2.78 mM in rat cells made by the perfusion method (cumulative curves) and was significantly greater than that in rabbit (1.4 mM). Maximum rates of contraction for rat cells averaged 4.59 micron/s in 8 mM calcium. Rat cells contracted faster than they relaxed, whereas rabbit cells in 8 mM calcium relaxed faster than they contracted. Rat cells, maximally activated by either calcium or isoproterenol, contracted significantly faster than rabbit. There was no difference in rates of contraction (or relaxation) between rat cells prepared by perfusion and those made from chunks of tissue.

A protective effect of a mild acidosis on hypoxic heart muscle.

Isolated spontaneously beating and electrically-paced rabbit hearts were perfused aerobically and then with hypoxic buffer (PO2 < 25 mmHg) at pH 7.4, 6.9 and 6.6, the required pH being established by varying the CO2 content of the gas mixture bubbled through the perfusion buffer. After the required period of hypoxic perfusion the hearts were either reoxygenated with buffer solution at pH 7.4 or they were assayed for ATP and creatine phosphate or their mitochondria were harvested and assayed for QO2, respiratory control index and Ca2+-accumulating activity. Resting and peak developed tension were monitored continuously. n nHearts made hypoxic at pH 7.4 exhibited reduced ATP and creatine phosphate content. The respiratory activity of their mitochondria was low and their Ca2+-accumulating activity high relative to that obtained for hearts made hypoxic at pH 6.9. n nThere was a greater recovery of tension development during reoxygenation of hearts made hypoxic at pH 6.8, relative to recovery obtained for the pH 7.4 hypoxic hearts. Perfusion of pH 6.9 also resulted in a relatively smaller rise in resting tension relative to that found during hypoxic perfusion at 7.4. n nThese results have been interpreted to mean that a mild respiratory acidosis has a protective effect on hypoxic heart muscle. This protective effect was not evident if the pH of the hypoxic buffer was decreased further, to 6.6.

Contamination of a cardiac sarcolemmal preparation with endothelial plasma membrane

Preparation of sarcolemma from whole rabbit heart using the method of Jones et al. (Jones,L.R., Besch, H.R., Fleming, J.W., McConnaughey, M.M. and Watanabe, A.M. (1979) J. Biol. Chem. 254, 530-539) results in a 46-fold purification of the endothelial plasmalemma-specific marker angiotensin converting enzyme. This implies contamination of the sarcolemma with vascular endothelial plasmalemma. During preparation of sarcolemma from sheep heart, using the same method, angiotensin converting enzyme copurified with the general plasma membrane marker (Na+ + K+)-ATPase. The ratio of myocyte to endothelial plasma membrane in the final preparation is therefore similar to that in the whole heart homogenate. Ultrastructural analysis has shown that the myocyte/endothelial surface area is 70:30 in whole cardiac muscle. Comparison of angiotensin converting enzyme activity of an endothelial plasma membrane fraction with that of whole heart sarcolemma suggests an upper limit of 42% for endothelial contamination. Contamination by endothelial plasmalemma was dramatically reduced by preparing sarcolemma from myocytes produced by proteolytic disruption of whole hearts. Following disruption, myocytes were separated from non-muscle cells by sedimentation through 0.5 M sucrose. Sarcolemma prepared from sheep cardiac myocytes had approximately 15-fold less angiotensin converting enzyme activity than whole sheep heart sarcolemma but comparable ouabain-inhibitable (Na+ + K+)-ATPase activity.

Myocardial Cell Abnormalities in Heart Failure: Experience from Studies on Single Myocytes

In the majority of patients with heart failure the underlying cause is one of myocardial dysfunction, which may in itself be either primary in origin or secondary to diseases of the coronary arteries or of the heart valves. In terms of cellular structure and function, myocardial failure can be due to abnormalities in the myocytes themselves or to abnormalities in the surrounding extracellular matrix. Histological studies in ischemic and dilated cardiomyopathy have revealed a spectrum of histological changes, including collagen accumulation, fibrosis, loss of myofilaments, myocyte slippage, myocyte hypertrophy, and cell death [1–3]. Although these changes alone have been postulated to produce progressive myocardial failure due to disruption of ventricular geometry [4], there is also evidence of a substantial functional impairment of the cardiac myocytes. In this chapter we review the evidence for the role played by this impairment of myocyte function, which has been identified from studies using isolated cardiac myocytes.

Advances in Myocardiology

In this essay, I take the liberty of doubting the widely held view that congestive cardiac failure is due to an inability of the heart to provide enough oxygen for the needs of the body. Instead, the syndrome is best explained by an inappropriate and prolonged stimulation of the neurohumoral defense reaction that developed during evolution to support exercise and preserve life. DIFFUSION AND CONVECTION Theology and science alike are agreed that life had its origins in the sea. In these primeval waters, the first cells floated freely. Special devices were developed to transport molecules across the cell wall, but the movement of substances toward and away from these was by diffusion, aided by the random currents of the outside world. As multicellular organisms developed and increased in size, they needed to impose their own convective streams on the surrounding water in order to ensure an exchange of materials that could no longer be achieved by simple diffusion Many simple organisms, such as the sponges and coelenterates, use the water in which they live as the sole means of convective transport. This system serves the combined functions of respiration, ingestion, and excretion. In more complex organisms, the transport distances within the body became too great to be maintained without the development of a mechanically propelled internal convective system. In the fishes, the function ofthe external convective system is still mixed. In addition to being used for the exchange of respiratory gases, it is used for the excretion of ammonia, the chief end-product of nitrogenous metabolism, and, to a variable degree, for ingestion. The motility of the bodyas a whole is also a mechanism of external convection. This is so even in a tmicellular organism such as paramecium. Does it just stir the water, or is it swimming? And what is the purpose of locomotion except to renew the environment? Many fishes have to swim in order to ventilate the gills. Predators rely on movement to capture food.