ELECTROPHYSIOLOGICAL PROPERTIES OF NEONATAL MOUSE CARDIAC MYOCYTES INPRIMARY CULTURE

1. The increasing utility of transgenic mice in molecular studies of t he cardiovascular system has motivated us to characterize the ionic cu rrents in neonatal mouse ventricular myocytes. 2. Cell capacitance mea surements (30 +/- 1 pF, n = 73) confirmed visual impressions that neon atal mouse ventricular myocytes in primary culture are considerably sm aller than freshly isolated adult ventricular myocytes. With the use o f electron microscopy, mitochondria and sarcoplasmic reticulum were fo und in close association with myofibrils, but transverse tubules were not observed. 3. Action potential durations, measured at 50 and 90% re polarization, were 23 +/- 1 and 42 +/- 2 ms respectively (n = 46). App lication of 4-aminopyridine (4-AP; 5 mM) prolonged action potential du ration at 50% repolarization by 26 +/- 5% (n = 3). The brevity of the action potential is explained by the rapid activation of a transient o utward K+ current upon voltage-clamp depolarization to plateau potenti als. 4. Potassium currents identified include an inward rectifier, a l arge 4-AP-sensitive transient outward, a slowly inactivating 4-AP-inse nsitive outward, a slowly activating delayed rectifier and a small rap idly activating E-4031 (10 mu M)-sensitive delayed rectifier K+ curren t. 5. Sodium currents (-305 +/- 50 pA pF(-1), n = 21) were recorded in 40 mM Na+ with Ni2+ (1 mM) to block Ca2+ currents and with K+ replace d by Cs+. The relative insensitivity of the Na+ current to block by te trodotoxin (IC50 = 2.2 +/- 0.3 mu M, n = 4) is distinctive of the card iac Na+ channel isoform. 6. Nitrendipine-insensitive (10 mu M) Ba2+ cu rrents elicited during steps from -90 to -30 mV measured -25 +/- 5 pA pF(-1) (n = 7, 30 mM Ba2+). Decay of these currents was complete durin g 180 ms depolarizations, even with Ba2+ as the charge carrier. These currents were not present when the holding potential was set at -50 mV . These data support the presence of a low threshold, T-type Ca2+ curr ent. 7. The maximal nitrendipine-sensitive L-type Ca2+ current density was -10 + 2 pA pF(-1) (n = 8) in 2 mM Ca2+ and -38 +/- 5 pA pF(-1) (n = 9) in 30 mM Ba2+. Exposure to isoprenaline (1 mu M) resulted in an 82% increase (n = 3) in the amplitude of the Ba2+ currents elicited at 0 mV. 8. Neonatal mouse cardiac myocytes in primary culture possess s urprisingly large inward currents given the brevity of their action po tentials. Their small cell size and apparent absence of transverse tub ules makes them technically favourable for whole-cell patch clamp stud ies. For these reasons, this preparation promises to be useful for the phenotypic characterization of transgenic mice.