EFFECTS OF LUMINAL VASOPRESSIN ON INTRACELLULAR CALCIUM IN MICROPERFUSED RAT MEDULLARY THICK ASCENDING LIMB

Recent evidence suggests that vasopressin exerts a regulatory influenc e on transport processes in the rabbit cortical collecting duct via bo th the basolateral and luminal membranes. The present study was undert aken to examine whether luminal vasopressin receptors, coupled to chan ges in intracellular calcium, were also present in microperfused rat m edullary thick ascending limb (mTAL), a key element of the urine conce ntrating mechanism. Addition of 1 nM vasopressin to the luminal microp erfusate elicited a small but significant and sustained rise in intrac ellular calcium, from basal values of 100.1+/-20.1 to 169.6+/-24.1 nM after 250 s. The effect observed following luminal addition of vasopre ssin was dose-dependent, with a larger increment of 190.2+/-32.2 nM ev oked by addition of 1 mu M vasopressin. Addition of 1 nM oxytocin to t he lumen did not cause a significant increase in intracellular calcium concentration, consistent with the response to vasopressin being medi ated by specific luminal vasopressin receptors. In the absence of calc ium in the bath and lumen together or in the bath alone, a residual re sponse to 1 mu M luminal vasopressin was still evident, suggestive of a small component of release of calcium from intracellular stores. Sel ective calcium removal from the luminal microperfusate alone left the response intact. These data are congruous with a model of vasopressin- induced entry of calcium which occurs via the basolateral membrane fol lowing ligand binding to the apical membrane. These findings, coupled with earlier observations in the collecting duct, suggest that a funda mental re-assessment of where and how vasopressin, and perhaps other h ormones, acts in the kidney may be required. Hormonal regulation of di stal tubular function may not, therefore, be determined only by bloodb orne delivery of hormone, but may also involve tubular fluid delivery to apical receptors in distal nephron sites.