Swelling-activated Gd(3+)-sensitive Cation Current and Cell Volume Regulation in Rabbit Ventricular Myocytes

The role of swelling-activated currents in cell volume regulation is unclear. Currents elicited by swelling rabbit ventricular myocytes in solutions with 0.6–0.9× normal osmolarity were studied using amphotericin perforated patch clamp techniques, and cell volume was examined concurrently by digital video microscopy. Graded swelling caused graded activation of an inwardly rectifying, time-independent cation current (I(Cir,swell)) that was reversibly blocked by Gd(3+), but I(Cir,swell) was not detected in isotonic or hypertonic media. This current was not related to I(K1) because it was insensitive to Ba(2+). The P(K)/P(Na) ratio for I(Cir,swell) was 5.9 ± 0.3, implying that inward current is largely Na(+) under physiological conditions. Increasing bath K(+) increased g(Cir,swell) but decreased rectification. Gd(3+) block was fitted with a K (0.5) of 1.7 ± 0.3 μM and Hill coefficient, n, of 1.7 ± 0.4. Exposure to Gd(3+) also reduced hypotonic swelling by up to ∼30%, and block of current preceded the volume change by ∼1 min. Gd(3+)-induced cell shrinkage was proportional to I(Cir,swell) when I(Cir,swell) was varied by graded swelling or Gd(3+) concentration and was voltage dependent, reflecting the voltage dependence of I(Cir,swell). Integrating the blocked ion flux and calculating the resulting change in osmolarity suggested that I(Cir,swell) was sufficient to explain the majority of the volume change at –80 mV. In addition, swelling activated an outwardly rectifying Cl(−) current, I(Cl,swell). This current was absent after Cl(−) replacement, reversed at E(Cl), and was blocked by 1 mM 9-anthracene carboxylic acid. Block of I(Cl,swell) provoked a 28% increase in swelling in hypotonic media. Thus, both cation and anion swelling-activated currents modulated the volume of ventricular myocytes. Besides its effects on cell volume, I(Cir,swell) is expected to cause diastolic depolarization. Activation of I(Cir,swell) also is likely to affect contraction and other physiological processes in myocytes.