Vasoconstrictors activate the Na+-K+-2Cl(-) cotransporter NKCC1 in rat aortic smooth muscle, but the mechanism is unknown. Efflux of Rb-86(+) from rat aorta in response to phenylephrine (PE) was measured in the absence and presence of bumetanide, a specific inhibitor of NKCC1. Removal of extracellular Ca2+ completely abolished the activation of NKCC1 by PE. This was not due to inhibition of Ca2+-dependent K+ channels since blocking these channels with Ba2+ in Ca2+-replete solution did not prevent activation of NKCC1 by PE. Stimulation of NKCC1 by PE was inhibited 70% by 75 muM ML-9, 97% by 2 muM wortmannin, and 70% by 2 mM 2,3-butanedione monoxime, each of which inhibited isometric force generation in aortic rings. Bumetanide-insensitive Rb+ efflux, an indication of Ca2+-dependent K+ channel activity, was reduced by ML-9 but not by the other inhibitors. Stretching of aortic rings on tubing to increase lumen diameter to 120% of normal almost completely blocked the stimulation of NKCC1 by PE without inhibiting the stimulation by hypertonic shrinkage. We conclude that activation of the Na+-K+-2Cl(-) cotransporter by PE is the direct result of smooth muscle contraction through Ca2+-dependent activation of myosin light chain kinase. This indicates that the Na+-K+-2Cl(-) cotransporter is regulated by the contractile state of vascular smooth muscle.