In the present paper, we examined a mechanism of the papaverine-induced relaxation in the smooth muscle of non-pregnant rat uterus. The hyperosmotic 65 mM KCl (H-65K⁺)-or oxytocin-induced contraction in the uterus was inhibited by an addition of papaverine in a concentration-dependent manner. Papaverine did not increase both cAMP and cGMP contents in the uterus in the presence of H-65K⁺ or oxytocin. In fura 2 loaded muscles, papaverine did not affect an increase of [Ca²⁺]_i level by high K⁺ or oxytocin. In permeabilized muscles, papaverine had no effect on the Ca²⁺-induced contraction. H-65K⁺ and oxytocin increased the rate of oxygen consumption 1.8 and 1.5 times higher than that in the resting condition, respectively. The increase of oxygen consumption in the H-65K⁺ or oxytocin was significantly inhibited by papaverine (1-100 μM). These results suggested that papaverine inhibits smooth muscle contraction mainly by inhibition of mitochondrial respiration in rat uterus as well as guinea pig ileum, which shows a highly spontaneous activity and a highly metabolic dependency of a contraction.

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The mechanisms of the β-adrenoceptor-mediated relaxation induced by epinephrine in guinea pig taenia caecum were examined. The relaxant response to epinephrine was unaffected by propranolol (∼10^-5M) or phentolamine (∼10^-5M). The response to epinephrine was antagonized in a concentration-dependent manner by bupranolol, and Schild plot of the data revealed the pA₂ value of 5.87. Epinephrine significantly increased cyclic AMP level in this preparation. Bupranolol (10^-4M) significantly decreased the cyclic AMP level that was elicited by epinephrine, whereas propranolol (10^-5M) produced no effect. These results suggest that the relaxant response to epinephrine in the guinea pig taenai caecum is mainly mediated by β₃-adrenoceptors.

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Diverse expression of voltage dependent K⁺ (Kv) channels was examined in smooth muscles (SMs) ; carotid artery (CA), mesenteric artery (MA), urinary bladder (UB), and vas deferens (VD) of the rat, using RT-PCR based analyses. Among eight Kv channel subtypes examined (Kv 1.1, Kv 1.2, Kv 1.5, Kv 1.6, Kv 2.1, Kv 2.2, Kv 3.1, and Kv 3.2), expression of three delayed rectifier Kv (K_D) channel (Kv 1.2, Kv 1.5, and Kv 2.1) transcripts was observed in these SMs. To determine precisely the expression levels of the transcripts encoding K_D subtypes, those of three K_D subtypes (Kv 1.2, Kv 1.5, and Kv 2.1) were determined by competitive PCR. In vascular SM tissues, CA and MA, Kv 1.2 and Kv 1.5 transcripts were expressed at relatively high levels, whereas in visceral SM tissues, UB and VD, Kv 2.1 transcripts were expressed at the relatively high levels. These results suggest that the diverse expression of K_D subtypes is, at least in part, responsible for differences in electrical excitability and also for the variation of the electrophysiological and pharmacological phenotypes as tonic and phasic SMs.

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