The activity of volume-sensitive Na⁺,K⁺,Cl^– cotransport (NKCC) is increased in several experimental models of primary (essential) hypertension. This study elucidates the role of cell volume in contractions of endothelium-denuded vascular smooth muscle rings (VSMR) from the rat aorta. Both swelling in hyposmotic medium as well as hyper- and isosmotic shrinkage by the addition of sucrose and transfer from hypo- to isotonic solution, respectively, led to VSMR contractions. Swelling-induced contractions were accompanied by activation of Ca²⁺ influx and were abolished by nifedipine and verapamil. In contrast, contractions of shrunken cells were insensitive to the presence of L-type channel inhibitors and occurred in the absence of Ca²⁺_o. Thirty-min preincubation with 100 µM bumetanide, a potent and selective NKCC inhibitor, decreased Cl^–_i content, nifedipine-sensitive ⁴⁵Ca uptake and contractions triggered by modest depolarization ([K⁺]_o = 30 mM). Elevation of [K⁺]_o to 60 mM completely abolished the effect of bumetanide on these parameters. At 100 µM, bumetanide almost completely abrogated phenylephrine-induced contraction, partially suppressed contractions triggered by hyperosmotic shrinkage, but potentiated contractions of isosmotically-shrunken VSMR. Our results strongly suggest that bumetanide inhibits contraction of modestly-depolarized cells via Cl^–_i-mediated membrane hyperpolarization and augments contraction in isosmotically-shrunken VSMR via suppression of NKCC-mediated regulatory volume increase. The mechanism of bumetanide’s inhibitory action on contraction of phenylephrine-treated and hyperosmotically-shrunken VSMR should be examined further.

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