Effects of cholinergic agents on the vasopressin-mediated water transport in the isolated toad bladder

The aim of this work was to study the effect of some pharmacological cholinergic agents on the events that follow the interaction of arginine vasopressin with toad bladder membrane receptors related to synthesis of 3′5′_cAMP. The water flow through the membrane was measured gravimetrically in sac preparations of the membrane. In the absence of arginine vasopressin (AVP), carbachol induced a significant increase in the water flow (37%) related to the basal (Ringer's solution). On the other hand, when carbachol and AVP were associated, a significant decrease of AVP hydrosmotic activity occurred (23%). The inhibitory effect of carbachol on the AVP action was almost completely abolished by the cholinergic antagonists atropine, pirenzepine, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) and the calcium antagonist lanthanum. Similarly, when carbachol and 3′5′ cyclic adenosine monophosphate (3′5′_cAMP) were associated, a decrease of nucleotide hydrosmotic activity was observed (12.80%). This effect was partially restored by the addition of pirenzepine or 4-DAMP in the bath solution. These results suggest a role for muscarinic receptors of sub-type M₁ and M₃, which are involved in the intracellular calcium release. The increase of calcium concentration in the intracellular medium acts as a negative modulator in the hydrosmotic action of antidiuretic hormone.