It has been known for over 100 years that acidosis decreases the contractility of cardiac muscle. However, the mechanisms underlying this decrease are complicated because acidosis affects every step in the excitation-contraction coupling pathway, including both the delivery of Ca2+ to the myofilaments and the response of the myofilaments to Ca2+. Acidosis has diverse effects on Ca2+ delivery. Actions that may diminish Ca2+ delivery include 1) inhibition of the Ca2+ current, 2) reduction of Ca2+ release from the sarcoplasmic reticulum, and 3) shortening of the action potential, when such shortening occurs. Conversely, Ca2+ delivery may be increased by the prolongation of the action potential that is sometimes observed and by the rise of diastolic Ca2+ that occurs during acidosis. This rise, which will increase the uptake and subsequent release of Ca2+ by the sarcoplasmic reticulum, may be due to 1) stimulation of Na+ entry via Na(+)-Ca2+ exchange; 2) direct inhibition of Na(+)-Ca2+ exchange; 3) mitochondrial release of Ca2+; and 4) displacement of Ca2+ from cytoplasmic buffer sites by H+. Acidosis inhibits myofibrillar responsiveness to Ca2+ by decreasing the sensitivity of the contractile proteins to Ca2+, probably by decreasing the binding of Ca2+ to troponin C, and by decreasing maximum force, possibly by a direct action on the cross bridges. Thus the final amount of force developed by heart muscle during acidosis is the complex sum of these changes.

Abstract-The transverse tubules (t-tubules) of mammalian cardiac ventricular myocytes are invaginations of the surface membrane. Recent studies have suggested that the structure and function of the t-tubules are more complex than previously believed; in particular, many of the proteins involved in cellular Ca 2ϩ cycling appear to be concentrated at the t-tubule. Thus, the t-tubules are an important determinant of cardiac cell function, especially as the main site of excitation-contraction coupling, ensuring spatially and temporally synchronous Ca 2ϩ release throughout the cell. Changes in t-tubule structure and protein expression occur during development and in heart failure, so that changes in the t-tubules may contribute to the functional changes observed in these conditions. The purpose of this review is to provide an overview of recent studies of t-tubule structure and function in cardiac myocytes. Key Words: cardiac muscle Ⅲ t-tubules Ⅲ excitation-contraction coupling Ⅲ heart failure T he transverse tubules (t-tubules) of mammalian cardiac ventricular myocytes are invaginations of the surface membrane that occur at the Z line and have both transverse and longitudinal elements. Many of the proteins involved in excitation-contraction coupling appear to be concentrated at the t-tubules. Therefore, it has been suggested that the t-tubules play a central role in cell activation. In the present review, we will consider the immunohistochemical and functional evidence for protein localization at the t-tubules, potential problems in the interpretation of such data, and the functional consequences of such localization. We will also consider the possible role of the t-tubules in the functional changes that occur during cardiac development, hypertrophy, and failure. Occurrence and Morphology of the T-TubulesT-tubules are present in the cardiac tissue of all species of mammals so far investigated (eg, mice, 1 rats, 2 guinea pigs, 3 rabbits, 4 dogs, 5 pigs, 1 and humans 6 ) but appear to be absent in avian, 7 reptile 8 and amphibian 8 cardiac tissue. Within mammalian cardiac tissue, t-tubules occur predominantly in ventricular myocytes, being either absent or far less developed in atrial, pacemaking, and conducting tissue, 9 although a recent report has suggested that Ϸ50% of atrial myocytes possess a sparse irregular tubular system. 10 The following discussion will concentrate on mammalian ventricular myocytes.The t-tubules are invaginations of the sarcolemma and glycocalyx, which appears to remain associated with the sarcolemma within the t-tubules. 11 Early studies of cardiac muscle showed that they occur at the Z line, at the end of each sarcomere 12 ; therefore, they occur at intervals of Ϸ2 m along the longitudinal axis of the ventricular myocyte. Subsequent studies have shown that the t-tubular system also has longitudinal extensions. 13 Although the t-tubules leave the surface membrane at the Z line, forming an approximately rectangular array, only Ϸ60% of the tubular volume occurs near the Z line; the other 40% ...

Formamide-induced osmotic shock has been used to detubulate isolated adult rat ventricular myocytes (i.e., disrupt the surface membrane-T tubule junction). Cell volume, calculated from cell length and width, rapidly decreased and increased upon application and removal of formamide, respectively. After treatment with formamide, membrane capacitance decreased by 26.4% (from 199.4 ± 18.7 pF in control cells to 146.7 ± 6.4 pF in formamide-treated cells; n = 13, P < 0.05). However, the amplitude of the L-type Ca2+ current ( I Ca) decreased by a greater extent (from 0.75 ± 0.14 to 0.18 ± 0.03 nA; n = 5, P < 0.05) so that the density of I Ca decreased by 74.5%. Simultaneous measurements of I Ca and Ca2+ transients (monitored using fura 2) showed that both decreased rapidly upon removal of formamide. However, the Ca2+ content of the sarcoplasmic reticulum showed little change. Cross-striations, visualized with the fluorescent dye di-8-aminonaphthylethenylpyridinium, were sparse or absent in cells that had been treated with formamide, suggesting that formamide can successfully detubulate cardiac cells and that I Ca is concentrated in the T tubules, which therefore play an important role in excitation-contraction coupling.

There is good evidence that elevated [Ca2+]i, produced by an influx of Ca2+ in exchange for Na+, is the underlying pathology in reperfusion or reoxygenation damage. Further measurements of [Na+]i and [Ca2+]i during ischemia and reperfusion, coupled with information about metabolic levels, are needed to confirm or refute this hypothesis. Contributions to cell damage by other mechanisms, e.g., oxygen free radicals, certainly cannot yet be excluded.

SUMMARY1. 31P nuclear magnetic resonance was used to measure the relative concentrations of phosphorus-containing metabolites in Langendorff-perfused ferret hearts. Intracellular concentrations of inorganic phosphate ([Pi]i), phosphocreatine ([PCr]i), ATP ([ATP]i) and H+ (pHi) were monitored under control conditions and while oxidative phosphorylation and/or glycolysis were prevented. Mechanical performance was assessed by recording the pressure developed in a balloon placed in the left ventricle.2. Oxidative phosphorylation was prevented either by replacement of 02 with N2 or by addition of cyanide. When the rate of oxidative phosphorylation was reduced by either method, developed pressure fell to a stable level of about 35 % of control after 5 min. The pHi (control value 6 98) first increased to a peak of 7 07 after 2 min but then decreased to give a stable acidosis (pH 6 85).[PCr]i decreased rapidly to about 15 % of the control value after 5 min whereas [ATP]i declined very slowly, reaching about 90 % of the control value after 10 min.3. Reduction in the rate of glycolysis was achieved either (i) by removal of external glucose and depletion of glycogen stores by a long (1-2 h) period of stimulation or (ii) by removal of glucose and application of 2-deoxyglucose (1 mM) for 30-60 min.These procedures had only a small effect on pressure development, [ATP]i, [PCr]i and pHi. Measurements of lactate production showed that these procedures reduced the rate of glycolysis by a factor of about 10.4. When oxidative phosphorylation was prevented during periods when the rate ofglycolysis was reduced, developed pressure fell to less than 5 % of control after 5 min and there was a subsequent increase in resting pressure hypoxicc contracture). pHi (control value 7 03) first increased to a peak of 7-12 and then declined to about pH 7 00, but there was no subsequent acidosis. [PCr]i fell rapidly to about 10 % of control after about 5 min while [ATP]i declined to about half of its control value over 10 min.5. It is concluded that (i) when oxidative phosphorylation alone is prevented, the