Cell volume in vascular smooth muscle is regulated by bumetanide-sensitive ion transport

Vascular smooth muscle cells (VSMC) exhibit shrinkage-induced bumetanide-inhibited 86Rb influx and ethylisopropylamiloride (EIPA)-inhibited 22Na influx. In this study, we examined the role of these transport pathways in volume adjustment of VSMC after isosmotic and hyperosmotic shrinkage. Cell volume was assessed by measurement of [14C]urea distribution. An initial 18-20% cell volume decrease in isosmotically shrunken VSMC was followed by a regulatory volume increase (RVI). RVI was completely abolished by bumetanide but not by EIPA. No RVI was noted in hyperosmotically shrunken VSMC. The initial rate of bumetanide-inhibited 86Rb influx was two- to threefold higher in isosmotically shrunken VSMC than with hyperosmotic shrinkage. Hyperosmotic shrinkage of VSMC was accompanied by a three- to fourfold increase in the rate of bumetanide-inhibited 86Rb efflux, whereas isosmotic shrinkage augmented this component by only 20-30%. In contrast to bumetanide-inhibited 86Rb influx, isosmotic shrinkage slightly increased the rate of EIPA-sensitive 22Na influx. Hyperosmotic shrinkage led to transient activation of EIPA-inhibited 22Na influx, which was completely abolished in 15 min. Activation of adenosine 3',5'-cyclic monophosphate (cAMP) signaling with isoproterenol arborized VSMC and decreased their volume by approximately 15%. A similar volume decrease was seen in VSMC treated with the microfilament-disrupting compound, cytochalasin B. The isoproterenol-induced cell volume decrease was prolonged by the addition of bumetanide. Unlike isoproterenol, agents that raise intracellular Ca2+ (A-23187 and angiotensin II) did not modify VSMC volume. Thus our data demonstrate involvement of cAMP signaling in the regulating of VSMC volume and a key role of bumetanide-inhibited ion transport in the RVI after isosmotically induced shrinkage.