FACTORS INVOLVED IN THE RELAXATION OF FEMALE PIG URETHRA EVOKED BY ELECTRICAL-FIELD STIMULATION

1 Non-adrenergic, non-cholinergic (NANC) relaxations induced by electr ical field stimulation (EFS) were studied in pig isolated urethra. The mechanism for relaxation was characterized by measurement of cyclic n ucleotides and by study of involvement of different subsets of voltage -operated calcium channels (VOCCs). 2 EFS evoked frequency-dependent a nd tetrodotoxin-sensitive relaxations in the presence of propranolol ( 1 mu M), phentolamine (1 mu M) and scopolamine (1 mu M). At low freque ncies (<12 Hz), relaxations were rapid, whereas at high (>12 Hz) frequ encies distinct biphasic relaxations were evoked. The latter consisted of a rapidly developing first phase followed by a more long-lasting s econd phase. 3 Treatment with the NO-synthesis inhibitor N-G-nitro-L-a rginine (L-NOARG; 0.3 mM) inhibited relaxations at low frequencies of stimulation. At high frequencies (>12 Hz) only the first relaxation ph ase was affected. 4 Measurement of cyclic nucleotides in preparations subjected to continuous nerve-stimulation, revealed an increase in gua nosine 3':5'-cyclic monophosphate (cyclic CMP) levels from 1.3 +/- 0.3 to 3.0 +/- 0.4 pmol mg(-1) protein (P < 0.01). In the presence of L-N OARG, there was a significant decrease in cyclic GMP content to contro l. However, there was no increase in cyclic GMP content in response to EFS. Levels of cyclic AMP remained unchanged following EFS. 5 Treatme nt with the N-type VOCC-inhibitor, omega-conotoxin GVIA (0.1 mu M) red uced NO-dependent relaxations, the effect being most pronounced at low frequencies (1-4 Hz) of stimulation. The NO-independent second phase of the relaxation; studied in the presence of L-NOARG (0.3 mM) at 16-3 0 Hz, was however markedly reduced or abolished by omega-conotoxin GVI A. omega-Conotoxin MVIIC (1 mu M) or omega-agatoxin IVA (30 nM) had no effect on electrically evoked relaxations. 6 These results suggest th at NANC-nerve derived urethral relaxation in the pig consists of two a pparently independent components. One is mediated by NO and associated with an increase in cyclic GMP content. The other mediator is unknown and produces relaxations not associated with changes in levels of cyc lic nucleotides. The release of this mediator seems to involve the N-t ype VOCC, since the relaxation was markedly reduced or abolished by om ega-conotoxin GVIA.