Passive membrane properties, steady-state Na/K pump current (I p) and modulation of I p by adrenergic agonists were studied with patch-clamp techniques in adult rat ventricular myocytes that were freshly isolated or maintained in culture for 1–4 days. Freshly isolated (day 0) myocytes had a 1.7–1.8 times smaller specific membrane resistance compared with that of cells on any day in culture. From day 0 to 4 there was a progressive decrease in cell capacitance (−17.6 ± 0.8 pF/day) without a parallel decline in cell dimensions. The pump current density (1.55 pA/pF) was stable over the 0–4 days in culture. In rod-shaped myocytes norepinephrine (NE) and isoproterenol (ISO) stimulated I p in a dose-dependent manner, with an apparent affinity of 36 ± 8 and 1.5 ± 0.4 nm, and maximum stimulation of 0.65 ± 0.02 and 0.57 ± 0.02 pA/pF, respectively. Nadolol suppressed this effect, suggesting that it was mediated by β-adrenergic receptor activation. An inverse relationship was found between steady-state I p and the stimulation of I p by NE. In contrast to what was shown in guinea pig cardiac myocytes, in rat myocytes isoproterenol stimulation of I p was not increased by intracellular [Ca] and it did not change the I p -membrane potential relation. These results show that in adult rat cardiac myocytes NE and ISO are potent stimulators of Na/K pump activity, and this effect may be studied using rat myocytes maintained in short-term culture.
The Journal of Membrane Biology – Springer Journals
Published: Jun 1, 1998
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