Transgenic mice expressing atrial natriuretic factor-SV40 T-antigen fusion genes (ANF-TAG) developed unilateral right atrial tumors composed of differentiated dividing cardiomyocytes. The atrial tumors could be propagated as transplantable tumor lineages in syngeneic animals. Cardiomyocytes derived from ANF-TAG atrial tumors did not proliferate in tissue culture. However, cardiomyocytes derived from the transplantable tumor lines proliferated in culture, and these proliferating cardiomyocytes could be passaged in culture and recovered from frozen stocks. Cardiomyocytes from either tumor source were highly differentiated as determined by diverse functional and structural criteria. The cells continued to express numerous cardiac-specific proteins and retained ultrastructural features characteristic of cardiomyocytes including well-formed myofibrils, transverse tubules, and intercalated disks. In addition, the cultured cells displayed spontaneous electrical and contractile activities. These atrial tumor cardiomyocytes are a novel experimental resource for the identification of genes regulating the cardiomyocyte cell cycle.

The subcellular distribution of prolyl endopeptidase, and of cation-sensitive neutral endopeptidase, two enzymes actively metabolizing many neuropeptides, was determined in homogenates of rabbit brain. The subcellular distribution of both enzymes was more similar to lactate dehydrogenase, a cytoplasmic enzyme marker, than to choline acetyltransferase, a synaptosomal marker. Only 35% of the activity of these two neutral endopeptidases was found in the crude mitochondrial fraction (P2), the bulk of the remaining activity being associated with the high-speed supernatant. Prolyl endopeptidase and cation-sensitive neutral endopeptidase thus can be regarded as mainly cytoplasmic enzymes in the rabbit brain.

Ventricular arrhythmias that accompany myocardial infarction in dogs may be secondary to the altered electrophysiological properties of the subendocardial Purkinje fibers that survive 24 hours after the coronary occlusion. To better understand the ionic mechanisms that underlie the altered electrical activity of these fibers, we have dispersed, using an enzymatic technique, Purkinje cells from the subendocardium of the infarcted ventricle (IZPCs) and compared their electrical and structural properties to Purkinje cells dispersed from fiber strands ( S erious ventricular arrhythmias often occur in humans immediately after coronary artery occlusion and during the "in hospital" phase of patient therapy, as well as for weeks to years after the infarction. 1 -2 Several hypotheses maintain that the mechanisms of these cardiac arrhythmias can be understood in terms of specific alterations in cellular electrical activity that result from the myocardial infarction. 34Much of what is known about the cellular electrophysiological changes that occur during these phases of arrhythmias has been derived from experiments on isolated tissues, either excised at difFrom the Department of Pharmacology, College of Physicians and Surgeons, Columbia University, New York, New York.Supported by Grant HL-34477 from the National Institutes of Health, Bethesda, Maryland.Address for correspondence: Dr. Penelope Boyden, Department of Pharmacology, College of Physicians and Surgeons of Columbia University, New York, NY 10032.Received June 13, 1988; accepted April 17, 1989. ferent times after the coronary artery occlusion or taken from normal hearts and superfused with solutions altered in such a way as to mimic conditions in ischemic hearts. Microelectrode studies on the isolated canine myocardium have shown 1) that Purkinje fibers survive on the endocardial surface of infarcts after the muscle has died; 2) that marked changes in the transmembrane action potentials of these surviving subendocardial Purkinje fibers occur at the time when the delayed phase of ventricular arrhythmias occurs; and 3) that the changes in the membrane potentials of these Purkinje fibers could cause arrhythmias. 5 " 7 Specifically, the maximum diastolic potential, action potential amplitude, and maximum rate of depolarization are reduced in Purkinje fibers in the subendocardium of the ventricles 24 hours after the coronary artery occlusion when compared with control subendocardial fibers.

Calcium ion-selective microelectrodes made with Simon's neutral carrier were used to measure simultaneously sarcoplasmic Ca2+ activity (aiCa) and resting tension (Tr) of rabbit ventricular muscle during reduction and restoration of external sodium ion concentration, [Na]0. Under the same experimental conditions the change in contractile tension (Ta) also measured. In resting muscle the aiCa was 38 +/- 17 nanomolar (mean +/- standard deviation; N = 10). The reduction of [Na]O from 153 to 20 millimolar led to about a threefold increase in aiCa with parallel increases in Tr and Ta. The time course of the change in aiCa was similar to that of the changes in Tr and Ta. The results are consistent with an important role of the sodium-calcium exchange system for regulating sarcoplasmic Ca2+ activity.

Combined with the known inhibitory effect of ruthenium red on mitochondrial calcium uptake, these data suggest that an important component of myocardial injury following ischaemia and reperfusion in the isolated rat heart is the result of mitochondrial calcium accumulation.