A novel sustained inward current activated at the diastolic pacemaker potential

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To clarify the relationship between the ionic current flow and the cell volume change, the cell width was measured using an optic device during patch clamp experiments in guinea-pig ventricular myocytes. On switching the isotonic bath solution to 50,70,150 and 200% osmotic solutions, the cell width changed to 121.1(]SY.+-。[)3.3% (n=4), 110.8(]SY.+-。[)2.2% (n=27), 87.1(]SY.+-。[)1.8% (n=6), 82.6(]SY.+-。[)1.1% (n=6) of control, respectively. The applicaation of 300 nM isoprenaline (Iso) in the 70% hypotonic solution induced a decrease of the cell width from 111.2(]SY.+-。[)1.4% to 106.2(]SY.+-。[)1.8%, (n=13). Following the Iso-induced volume decrease, returning to the isotonic solution induced a decrease of the cell width below the control level (94.8(]SY.+-。[)1.6%). The application of Iso in the isotonic solution also decreased the cell width to 96.5(]SY.+-。[)1.3% (n=8) control, accompanied by a membrane depolarization of 2-4 mV.In other 5 cells neither Iso-induced volume decrease nor membrane depolarization was observed. Under the ruptured whole-cell voltage clamp condition, Iso activated the inward Cl<@D1-@>D1 current and decreased the cell width. If a large outward or inward K<@D1+@>D1 current was induced by shifting the holding potential and by applying 200 muM pinacidil, the cell width decreased or increased, respectively. Under the gramicidin-perforated whole-cell clamp condition, the cell width did not change even when a large inward K<@D1+@>D1 current was induced. When the application of test solution was limited to a half of the elongated myocyte by using a micro-pipette, the cell width increased or decreased in the part exposed to the hypotonic or hypertonic test solutions, respectively. While the rest part of the myocyte responded in the opposite direction. It is concluded that a continuous ionic flux through ion channels is capable of inducing cell volume change.