Enhanced exocytotic release of norepinephrine consequent to nerve stimulation by low concentrations of cyclic nucleotides in the presence of phenoxy-benzamine

Abstract

Stimulation at 5 Hz of postganglionic nerves to the isolated guinea-pig heart which had been labeled with [3H]norepinephrine (NE) resulted in a proportional release of NE, total 3H, [3H]NE, and dopamine beta-hydroxylase. Phenoxybenzamine (3 micrometer) caused a significant increase in the release of all these indices of neurotransmitter release throughout a series of four consecutive stimulations. Stimulation in the presence of dibutyryl cyclic adenosine 3':5'-monophosphate (dibutyryl cyclic AMP) (1 X 10(-9) M) and 8-bromo cyclic guanosine 3':5'-monophosphate (8-bromo cyclic GMP) (1 X 10(-8) M) failed to alter any of the measured indices of release when compared with control. However, when perfused in combination with phenoxybenzamine, both cyclic nucleotide analogs significantly increased total 3H, [3H]NE, NE and dopamine beta-hydroxylase outflow with stimulation, as compared with phenoxybenzamine alone. Lower concentrations of both agents (1 X 10(-10) M dibutyryl cyclic AMP and 1 X 10(-9) M 8-bromo cyclic GMP) were less effective in augmenting release. The increased release of NE with nerve stimulation in the presence of phenoxybenzamine alone and with both phenoxybenzamine and the cyclic nucleotide analogs was associated with a significant increase in intraventricular pressure. In contrast, only the combination of 1 X 10(-9) M dibutyryl cyclic AMP plus phenoxybenzamine resulted in an increase in heart rate. The results suggest that phenoxybenzamine and the cyclic nucleotides, probably operating by two distinct mechanisms to enhance neurally mediated release, can act in concert to enhance neurotransmitter release when subeffective concentrations of cyclic nucleotides are used in conjunction with an effective concentration of phenoxybenzamine.