Mechanisms of relaxation induced by activation of β‐adrenoreceptors in smooth muscle cells of the guinea‐pig mesenteric artery

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Abstract
Relaxation of smooth muscle cells induced by activation of .beta.-adrenoceptors was investigated in intact and skinned muscles of the guinea pig mesenteric artery. In concentrations > 10-7 M, isoprenaline reduced the resting tone of intact preparations and also the amplitude of K contractions. When Ca was applied after previous superfusion with Ca-free solution, the amount of Ca accumulated into storage sites was increased by isoprenaline in polarized and depolarized ([K]o 128 mM) muscles. The amount of Ca stored increased even further when procaine and isoprenaline were applied simultaneously during store loading. Isoprenaline increased the concentration of cAMP as determined by radioimmunoassay. Application of isoprenaline at a concentration of 10-7 M increased cAMP from 2.2 .+-. 0.3 to 2.8 .+-. 0.6 pmol/mg wet wt and at 10-6 M increased it to 4.5 .+-. 0.8 pmol/mg wet wt after 5 min incubation (n = 4). Application of cAMP (3 .times. 10-6 M) with cAMP-dependent protein kinase (50 .mu.g/ml) had no effect on the pCa tension relationship in the skinned muscles. An increased concentration of cAMP (> 10-5 M) suppressed the Ca-induced concentration only in the presence of protein kinase. This protein kinase (50 .mu.g/ml) alone had no effect on the Ca-induced contraction. In skinned fibers, the Ca store was loaded by applying low concentrations of Ca. If cAMP (3 .times. 10-6 M) with protein kinase (50 .mu.g/ml) was applied during the loading procedure, the amount of Ca accumulated by the store increased if the loading solution contained 10-6 M-Ca applied for 2 min or less, but if the loading solution was applied for 3 min, or if higher Ca concentrations were used, the presence of cAMP with protein kinase decreased the store size, suggesting that a Ca-induced Ca-release mechanism was being activated. In skinned muscles, accumulation of Ca into the store site in the presence of cAMP (3 .times. 10-6 M) with protein kinase (50 .mu.g/ml) was further accelerated by simultaneous applications of procaine (5 mM), as here the Ca-induced Ca-release mechanism was suppressed. Activation of .beta.-adrenoreceptors by isoprenaline may increase the amount of cAMP in the intact muscles and lead to an increase in Ca accumulation into the store site. In the skinned muscles, the Ca-induced Ca-release mechanism was activated by cAMP and the Ca receptor for contraction (leiotonin C or calmodulin) was somewhat suppressed. These effects of exogenously applied cAMP require the presence of protein kinase. The relaxation following .beta.-adrenoceptor activation was more likely to involve Ca extrusion from the cell and accumulation of Ca in internal storage sites than suppression of the binding of calmodulin with the myosin light chain kinase.

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