Propagation of impulses in the guinea-pig ureter and its blockade by calcitonin gene-related peptide (CGRP)

Abstract

The guinea-pig ureter was placed in a three-compartment organ bath to enable the application of electrical stimuli or drugs to its renal end (R site), the middle region (M-site) or the bladder end (B-site) while recording mechanical activity at the R- and B-sites. All experiments were performed in ureters pre-exposed to capsaicin (10 μM for 15 min) to prevent the release of sensory neuropeptides from afferent nerves. Electrical field stimulation (EFS, 5–25 ms pulse width, 20 V) produced a phasic contraction at the site of stimulation (‘direct’ response to EFS) which propagated to the other end of the ureter. Section of the ureter at the M-site abolished the propagated response to EFS; after section, EFS applied at the M-site induced a phasic contraction at both the R-and B-sites. Likewise, the application of KCl at the M-site produced phasic contractions at both the R- and B-sites. Tetrodotoxin (1 μM), nifedipine (1 μM) or Bay K 8644 (1 μM) applied at the M-site had no influence on the direct or propagated responses to EFS; nifedipine (10 μM) applied at the M-site abolished the propagated responses without affecting the direct responses to EFS. Bay K 8644 (1 μM) applied at the R-site produced a marked enhancement of the direct response (EFS applied at R-site) while having no effect on the amplitude of the propagated response to EFS. Nifedipine (1 μM), applied at the R-site, produced a graded, time-dependent, inhibition of the direct response (EFS applied at R-site) and eventually suppressed it; the propagated response was unaffected until suppression of the direct response, when an allor-none blockade of the propagated response was observed. When applied at the B-site (EFS at Rsite), 1 μM nifedipine produced a graded, time-dependent, inhibition of the propagated response and eventually suppressed it, without affecting the direct response to EFS. For further pharmacological analysis of drug action on the propagated activity, EFS was applied at the R-site and drugs were applied at the M-site. Human αCGRP (CGRP, 0.1 μM) or cromakalim (1-3 μM) were applied in superfusion at the M-site in the absence or presence of glibenclamide (1 μM). Neither drug affected the direct response to EFS; both CGRP and cromakalim produced a reversible suppression of the propagated response. Glibenclamide suppressed the inhibitory activity of 1 μM cromakalim and partly antagonized the effect of CGRP; the blockade by glibenclamide was partly overcome by 3 μM cromakalim. The present findings are consistent with the idea that propagation of excitation occurs because of the spread of electrical activity between smooth muscle cells of the ureter and that conduction of impulses is independent of local changes in contractility. The present results also demonstrate that CGRP induced a conduction block along the ureter and that this effect involves activation of glibenclamide-sensitive K channels. Therefore, a local release of CGRP in response to pathophysiological stimuli is, in principle, capable of suppressing ureteral peristalsis and antiperistalsis.