Effect of Glyceryltrinitrate and 8–Br–cGMP on Tension and Phosphorylase a Activity in Vascular Smooth Muscle

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Abstract
The aim of the present study was to examine the effect of glyceryltrinitrate (GTN) and 8–Br–cGMP on tension and cytosolic calcium concentration in pre–contracted bovine mesenteric arteries (BMA). The activity of glycogen phosphorylase a was used as a measure of the cytosolic calcium concentration. The activity of this enzyme is regulated by the cytosolic calcium concentration and/or cAMP. Since the cAMP level was not found to be affected by GTN–treatment, the use of phosphorylase a activity to monitor changes in the cytosolic calcium concentration can be justified. The vessels were contracted with phenylephrine (10 μM) or 100 mM K+–depolarization, which caused an increase in phosphorylase a activity. Addition of 1 μM GTN to the phenylephrine–contracted vessels resulted in a 3–4–fold rise in intracellular cGMP level, which was accompanied by a large decrease in tension and phosphorylase a activity. The K+–depolarized vessels, on the other hand, were largely resistant to the relaxant action of GTN, and there was only a slight reduction of the phosphorylase a activity. In phenylephrine–contracted vessels, made tolerant to GTN by incubation at elevated pH in the presence of GTN (0.44 mM), no changes in tension and phosphorylase a activity were seen after stimulation with a test dose of GTN (1 μM). The cGMP response was also markedly blunted in the tolerant vessels. Relaxation of phenylephrine–contracted BMA induced by 8–Br–cGMP (0.5 mM) was also accompanied by a reduction in phosphorylase a activity. The close association between the relaxation of BMA and the reduction in phosphorylase a activity suggests that GTN and 8–Br–cGMP exert their effects mainly by reducing the cytosolic free calcium concentration although a contributing effect on the contractile apparatus can not be excluded