Isolated cells of the frog sinus venosus: properties of the inward current activated during hyperpolarization

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Abstract
Single sinus venosus cells from frog, Rana esculenta, were isolated using an enzymic dispersion procedure, involving applications of collagenase and protease. About 40%–60% of the cells showed spontaneous contractions. Isolated cells were studied in the whole-cell configuration. Regenerative action potentials were tetrodotoxin-insensitive and similar to those recorded in multicellular preparations. Hyperpolarizing pulses in the voltage range negative to −50 mV induced the activation of a time-dependent inward current, which was blocked by 4 mM caesium but less affected by barium ions. A lower concentration of caesium (1 mM) exerted a voltage-dependent reduction of the current and decreased the spontaneous pacing rate. The activation range of the hyperpolarization-activated current approximately extended from −50 mV to −110 mV, but varied from cell to cell. A high variability was observed in the behaviour of the activation kinetics. The current had a reversal potential near −20 mV that was shifted positively by increasing the external potassium concentration (from 3 mM to 30 mM) and negatively by reducing the external sodium concentration (from 115 mM to 30 mM). The hyperpolarization-activated inward current of the frog sinus venosus cell appears to be carried by both sodium and potassium ions. It shows electrophysiological properties similar to those of the I f current of the mammalian heart. The role of the current in the spontaneous activity is discussed.

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