xylan exohydrolase/xylosidase/xylobiase (EC 3.2.1.37). Although these three enzymes have been assigned the same number by the Enzyme Commission, we believe that the (1 + 4)-P-xylan exohydrolase might be classified separately.

The fate of gap junctions in dissociated adult myocytes, maintained for up to 22 hours in culture medium, was investigated by semiquantitative analysis of thin sections and by freeze-fracture electron microscopy. Gap junctions in the dissociated myocyte are intact bimembranous structures seen either as invaginated surface-located structures or as annular profiles in the cytoplasm. Surface-located junctions are sealed from the exterior by a sheet of nonjunctional membrane originating (together with the "outer" junctional membrane) from the formerly neighboring cell. Serial sectioning was used to establish that at least part of the annular gap junction population in the freshly isolated myocyte represents truly discrete cytoplasmic vesicles; thus, some gap junctions are rapidly endocytosed after myocyte separation. Analysis of the surfflce-located-to-annular gap junction ratio suggested that no further endocytosis occurred in rabbit and cat myocytes maintained for 22 and 15 hours, respectively. Guinea pig myocytes, by contrast, did appear to continue endocytosis in culture. Analysis of the distance of gap junctional structures from the cell surface suggested that little if any inward migration of gap junction vesicles occurred. Hypoxla had no detectable effect on the internalization or inward movement of gap junctions. The quantity of ultrastructurally detectable gap junction membrane appeared to remain constant over time, as did the incidence of"complex structures" (i.e., annular gap junction profiles with features previously suggested to represent degradation). New gap junction formation was negligible, and a reappraisal of the nature of "complex structures" led to the conclusion that the origin of these structures need not be related to degradation. Taken together, the findings suggest that degradation and disappearance of gap junctional membrane after isolation of the mature myocyte constitute a much slower process than previously believed, and the possibility that the cardiac gap junction protein has a longer half-life than its Address for correspondence: Dr. NJ. Severs, Department of Cardiac Medicine, National Heart and Lung Institute, Dovehouse Street, London SW3 6LY, England.Received January 7, 1988; accepted November 29, 1988. reflected in distinct differences in their structure, biochemistry, and composition. 5 - 7Refinements in techniques for the dissociation and culture of myocytes from the mature mammalian heart 8-11 have opened new opportunities for exploration of the specific properties of cardiac gap junctions. Isolated pairs of myocytes have already been exploited in electrophysiological and dye transfer studies 12 -14 ; other potential applications include use of dissociated and cultured myocytes for 1) the study of cardiac gap junction degradation and formation, and 2) as a starting material for the isolation of pure cardiac myocyte gap junctions from which contamination with junctions of other cell types in the heart (e.g., endothelial cells, smooth muscle cells, and myofibroblasts) can be e...

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Hypoxic-induced damage of rabbit heart muscle has been quantitated in terms of the release of intracellular enzymes (including creating phosphokinase, CPK) into the extracellular space, a gain in tissue Na+ and Ca2+, a loss of tissue K+, the depletion of the adenosine triphosphate (ATP) and creatine phosphate (CP) reserves, and ultrastructural damage. This ultrastructural damage incolves the disruption of the plasmalemma, swelling and distortion of the mitochondria, disruption of the myofilaments, and the development of contraction bands. In isolated Langendorff-perfused rabbit hearts perfused under either aerobie (pO2 greater than 80.0 kPa [600 mm Hg]) or hypoxie (pO2 less than 0.80 kPa [6 mmHg]) conditions, and either with or without glucose substrate, 0.5-1.0 mg/litre dl verapamil reduced the amount of ultrastructural damage caused by hypoxie perfusion. Verapamil (0.5-1.0 mg/litre) also reduced the rate at which the hypoxie muscle gained Na+ and lost K+; it reduced the rate at which the endogenous stores of ATP and CP were depleted and, provided that the extracellular phase contained Ca2+, it decreased the rate at which CPK appeared in the coronary effluent. Verapamil failed to prevent the hypoxie muscle from gaining Ca2+. These results are discussed in terms of a possible protective effect of dl verapamil on hypoxie heart muscle.