CALCITONIN-GENE-RELATED PEPTIDE AS INHIBITORY NEUROTRANSMITTER IN THE URETER

A dense plexus of calcitonin gene related peptide (CGRP) containing nerve fibres is present in the mammalian ureter, from which CGRP is released by depolarizing stimuli, including chemicals normally present in the urine. CGRP exerts a profound, receptor-mediated, inhibitory effect on the evoked motility of the ureter by suppressing latent pacemakers in the smooth muscle. This effect is largely glibenclamide sensitive, indicating the activation of potassium (K) channels in its genesis. Electrical stimulation of intramural nerves in the guinea-pig ureter produces a transient membrane hyperpolarization, which is blocked by glibenclamide or by capsaicin pretreatment, enhanced in a low-K medium, and inhibited by a CGRP receptor antagonist. Thus endogenous CGRP acts as a neurotransmitter K channel opener in the ureter. The refractory period of the guinea-pig ureter is markedly and similarly reduced by capsaicin pretreatment or administration of a CGRP receptor antagonist, indicating that endogenous CGRP can modulate the maximal frequency of ureteral peristalsis. Using a three-chamber organ bath that enabled the separate perfusion of the renal, middle, and bladder regions of the organ, evidence was obtained that CGRP blocks propagation of impulses along the ureter through a glibenclamide-sensitive mechanism. These findings indicate a role of CGRP in the local regulation of ureteral motility and peristalsis.