Hyperpolarization-activated inward current in embryonic chick cardiac myocytes: developmental changes and modulation by isoproterenol and carbachol

Abstract

Modulation of the hyperpolarization-activated inward current (I f) in embryonic chick ventricular myocytes was examined using whole-cell voltage-clamp. Long (3 s) hyperpolarizing pulses were applied from a holding potential of −30 mV to steps of −40 to − 120 mV. I f was marked in 3-day-old cells, diminished at 10 days, and was almost completely gone at 17 days. I f current density (at −120 mV) was −6.7 ± 1.3 (3 days), −3.3 ± 1.0 (10 days), and −2.0 ± 0.5 pA/pF (17 days). I f reduction paralleled the decrease in spontaneous activity. In 3-day cells, the threshold potential was −50 to −60 mV, and the reversal potential was −13.4 ± 1.3 mV. The time course of activation was fitted by a single exponential and was temperature dependent: τ was 1.3 ± 0.4 s at 20°C(and 0.7 ± 0.4 s at 30°C (at−120 mV). I f amplitude was enhanced by 12.1 ± 1.8% at 30 °C compared with 20°C. Cs + (3 mM) blocked I f and had a negative chronotropic effect (rate decreased by 61%). Isoproterenol (1 μM) caused a positive chronotropic effect (17.1 ± 2.9%) and increased I f by 65.2 ± 5.6%. Carbachol (0.1 μM) had a negative chronotropic effect (26.3 ± 3.4%), and decreased I f by 41.2 ± 1.3%; it also reversed the enhancement produced by isoproterenol. Intracellular application of 100 μM GTP-γS decreased basal I f by 35.2 ± 5.0%, but potentiated the stimulant effect of isoproterenol (by 37.8 ± 4.7%) and the inhibitory effect of carbachol (21.2 ± 4.3%). These results indicate that the I f current may contribute to the pacemaker potential of young embryonic chick heart cells and decreases during development. β-Adrenoceptor agonists stimulate I f, whereas muscarinic cholinergic agonists inhibit I f and reverse β-adrenoceptor stimulation. G proteins may directly and indirectly couple autonomic receptors to I f channels.