Involvement of cyclic nucleotides in prejunctional modulation of noradrenaline release in mouse atria

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Abstract
1 In mouse isolated atria previously incubated with [3H]-noradrenaline, 8-bromo-cyclic AMP (3–270 μm) produced a concentration-dependent increase in the fractional stimulation-induced outflow of radioactivity. 8-Bromo-cyclic GMP induced a lesser increase in the stimulation-induced outflow. 2 The phosphodiesterase inhibitors: M&B 22948 (90 μm); ICI 63197 (30 and 90 μm) and 3-isobutyl-1-methylxanthine (90 μm) increased the fractional stimulation-induced outflow. Together these results indicate that cyclic AMP may have a modulatory effect on noradrenaline release. 3 The inhibition of the stimulation-induced outflow produced by clonidine (0.03 μm) and its facilitation produced by phentolamine (1 μm) were unaltered in the presence of 8-bromo-cyclic AMP (90 μm). However, in the presence of 8-bromo-cyclic AMP (270 μm), the facilitatory effect of phentolamine was enhanced, but the inhibitory effect of clonidine (0.03 μm) was unaltered. In the presence of ICI 63197 (30 μm) the inhibitory effect of clonidine (0.03 μm) was unaltered, but the facilitatory effect of phentolamine (1 μm) was slightly enhanced. 4 Isoprenaline (0.003-0.1 μm) enhanced the fractional stimulation-induced outflow, an effect blocked by propranolol (0.1 μm). In the presence of 8-bromo-cyclic AMP (90 μm), the facilitatory effect of isoprenaline (0.01 μm) was blocked. In the presence of ICI 63197 (30 μm) the facilitatory effect of isoprenaline (0.003 μm) was potentiated. 5 These results suggest that whereas β-adrenoceptor-mediated enhancement of noradrenaline release is linked to the stimulation of adenylate cyclase and enhanced formation of cyclic AMP, α-adrenoceptor-mediated inhibition of noradrenaline release is not linked to inhibition of adenylate cyclase activity.

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