Regulation of cellular volume in rabbit ventricular myocytes: bumetanide, chlorothiazide, and ouabain

Abstract

Video microscopy was used to study the regulation of cell volume in isolated rabbit ventricular myocytes. Myocytes rapidly (less than or equal to 2 min) swelled and shrank in hyposmotic and hyperosmotic solutions, respectively, and this initial volume response was maintained without a regulatory volume decrease or increase for 20 min. Relative cell volumes (normalized to isosmotic solution, 1T) were as follows: 1.41 +/- 0.01 in 0.6T, 1.20 +/- 0.04 in 0.8T, 0.71 +/- 0.04 in 1.8T, and 0.57 +/- 0.03 in 2.6T. These volume changes were significantly less than expected if all of the measured volume was osmotically active water. Changes in width and thickness were significantly greater than changes in cell length. The idea that cotransport contributes to cell volume regulation was tested by inhibiting Na(+)-K(+)-2Cl- cotransport with bumetanide (BUM) and Na(+)-Cl- cotransport with chlorothiazide (CTZ). Under isotonic conditions, a 10-min exposure to BUM (1 microM), CTZ (100 microM), or BUM (10 microM) plus CTZ (100 microM) decreased relative cell volume to 0.87 +/- 0.01, 0.86 +/- 0.02, and 0.82 +/- 0.04, respectively. BUM plus CTZ also modified the response to osmotic stress. Swelling in 2.6T medium was 76% greater and shrinkage in 0.6T medium was 29% less than in the absence of diuretics. In contrast to the rapid effects of diuretics, inhibition of the Na(+)-K+ pump with 10 microM ouabain for 20 min did not affect cell volume in 1T solution. Nevertheless, ouabain decreased swelling in 0.6T medium by 52% and increased shrinkage in 1.8T medium by 34%. These data suggest that under isotonic conditions Na(+)-K(+)-2Cl- and Na(+)-Cl- cotransport are critical in establishing cell volume, but osmoregulation can compensate for Na(+)-K+ pump inhibition for at least 20 min. Under anisotonic conditions, the Na(+)-K+ pump and Na(+)-K(+)-2Cl- and/or Na(+)-Cl- cotransport are important in myocyte volume regulation.