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Properties of spontaneous depolarizations in circular smooth muscle cells of rabbit urethra

1 . Intracellular microelectrode recordings were made from circular smooth muscle of rabbit urethra. 2 . The smooth muscle of urethra was spontaneously active exhibiting large, regularly occurring depolarizations, termed slow waves (SWs), 1–3 s in duration, up to 40 mV in amplitude and generated every 3–15 s and small irregularly occurring events (or summations there of) termed spontaneous transient depolarizations (STDs) of < 1 s in duration. 3 . The SWs and STDs were not sensitive to 10−6 m atropine, 10−6 m phentolamine, 10−5 m guanethidine or 10−6 m tetrodotoxin, indicating that they were myogenic in origin. 4 . Application of 3 × 10−6 m nifedipine or 5 × 10−5 m GdCl3 did not inhibit the generation of SWs or STDs, indicating that activation of L‐type Ca2+ channels and non‐selective cation channels are not essential for their generation. However, the duration of SWs but not STDs was reduced by nifedipine, indicating L‐type Ca2+ channels contribute to the plateau‐like potential of SWs. 5 . Application of low chloride solution (6.4 mM), niflumic acid (10−5‐10−4 m) or 4,4′‐diisothiocyanostilbene‐2,2′‐disulphonic acid (DIDS, 10−4‐5 × 10−4 m) inhibited the generation of SWs and STDs, suggesting an involvement of chloride channels. 6 . Application of nominally Ca2+ free solution, 5 × 10−5 m BAPTA‐AM, 10−5 m cyclopiazonic acid, 10−2 m caffeine or 10−3 m procaine inhibited the generation of SWs and STDs, indicating that Ca2+ released from intracellular stores was required for the generation of SWs and STDs. 7 . Exogenously applied noradrenaline (10−7‐10−5 m) increased the frequency of SWs through stimulation of α‐adrenoceptors which was inhibited by sodium nitroprusside (SNP, 10−4 m). SNP also reduced the frequency of SWs without altering the membrane potential, an effect mimicked by 8‐bromocyclic GMP (10−3 m) indicating that SNP acted by elevating the production of cyclic GMP. 8 . It is concluded that smooth muscle cells of the rabbit urethra exhibit SWs and STDs which are likely to be induced by stimulation of Ca2+‐activated chloride channels evoked by release of Ca2+ from intracellular stores.