Protein kinase activity in isolated tubules of rat renal medulla

It has been recently documented that not only the medullary collecting tubule (MCT), but also the medullary thick ascending limb of Henle's loop (MAL) contains vasopressin- (VP) sensitive adenylate cyclase activity. According to a current view, an integral part of the renal tubular response to VP is an activation of cyclic 3',5'-AMP- (cAMP) dependent protein kinase (PK). Therefore, in the present study we measured PK activity, the in situ activation of PK, and cAMP content in isolated MCT and MAL microdissected from rat kidney. Both MAL and MCT contain cAMP-sensitive and cAMP-insensitive PK. In both segments stimulation of PK activity by cAMP was dose dependent; the maximal stimulation was achieved at 10^-6 M cAMP. Incubation of intact MCT and MAL with VP (10^-6M) resulted in a significant increase in PK activity and a significant increase in the (-cAMP/+cAMP) PK activity ratio. On the other hand, salmon calcitonin, which stimulated adenylate cyclase in the MAL but not in the MCT, also increased PK activity and the (-cAMP/+cAMP) PK activity ratio in the MAL but had no effect in the MCT. Incubation of isolated MCT and MAL segments with VP (10^-6M) elicited a significant increase in cAMP content in the MAL and, under the same conditions, a markedly higher elevation of cAMP content in the MCT. The results provide evidence that in both the MCT and MAL vasopressin results in the in situ activation of PK and an increase in cAMP levels, thus suggesting that these intracellular elements are an integral part of vasopressin action in the MAL and MCT.