Angiotensin receptor subtypes in thin and muscular juxtamedullary efferent arterioles of rat kidney

ANG II controls the vascular tone of pre- and postglomerular arterioles, and thereby glomerular filtration, through binding to either A[T.sub.1A], A[T.sub.1B], or A[T.sub.2] receptors. A[T.sub.1] receptors, which are coupled to intracellular [Ca.sup.2+] signaling, have vasoconstricting effects, whereas A[T.sub.2] receptors, whose signaling mechanism is unknown, induce vasodilatation. The angiotensin receptors have been characterized in afferent arterioles, which express the three types of receptors, but not in efferent arterioles. Two subpopulations of juxtamedullary efferent arterioles, muscular ones which terminate as vasa rectae and thin ones which terminate as peritubular capillaries, have been described. They display functional heterogeneity with regard to the ANG II response. To evaluate whether these differences are associated with differential expression of ANG II receptors, we examined the expression pattern of A[T.sub.1A], A[T.sub.1B], and A[T.sub.2] receptor mRNAs by RT-PCR in these arterioles and studied the effect of valsartan, a specific A[T.sub.1]-receptor antagonist. Results indicate that muscular arterioles express A[T.sub.1A], A[T.sub.1B], and A[T.sub.2] receptors, whereas thin arterioles only express the A[T.sub.1A], and A[T.sub.2] types, and at a much lower level. Valsartan fully inhibited ANG II-induced increases in intracellular [Ca.sup.2+] in both arteriolar types, but with different kinetics. In muscular arterioles, inhibition was monoexponential, whereas it displayed a marked positive cooperativity in thin arterioles. Finally, the apparent affinity for valsartan was higher in muscular than in thin arterioles. In conclusion, this study further documents the differences between muscular and thin efferent arterioles with regard to ANG II signalization in the rat kidney. angiotensin II; valsartan; calcium signaling