Effects of desipramine, trifluoperazine and other inhibitors of calmodulin on the secretion of catecholamines from the adrenal medulla and postganglionic sympathetic nerves of the salivery gland

Abstract

The effects of various drugs, known to be inhibitors of calmodulin, were tested on the secretion of catecholamines (CA) from the adrenal medulla and sympathetic nerves of the salivary gland of the rat. Secretion of CA from the perfused adrenal gland was evoked by injection of acetylcholine (ACh, 50 μg), excess K (700 μg), or transmural stimulation of splanchnic nerves (300 pulses at 10 Hz). Release of ³H-noradrenaline in the perfused salivary gland was evoked by transmural stimulation at 3 Hz for 30 s. CA secretion was reduced in a dose-dependent manner by 0.3 μM to 10 μM desipramine or imipramine. The effect of low doses (0.3 μM) was more pronounced on the secretion evoked by ACh and splanchnic nerve stimulation than that by excess K. The inhibition was independent of the frequency of nerve stimulation. Trifluoperazine (10–100 μM) and chlorpromazine (10–100 μM) reduced CA secretion evoked by all of the three procedures. The inhibitory effects of desipramine, trifluorperazine and chlorpromazine were completely reversed within 1 h after their washout. Secretion of CA obtained after reintroduction of Ca to the adrenal gland previously perfused with Ca-free medium was not blocked by desipramine or trifluoperazine. In fact, these agents markedly enhanced the secretory response. None of the drugs enhanced spontaneous secretion of CA from the adrenal gland during the nonstimulation period. Adrenal medullary cells accumulated significant amounts of Ca⁴⁵ (0.88 pg/mg) after stimulation with ACh. One μM desipramine, which reduced CA secretion over 60%, had no significant effect on ACh-induced accumulation of Ca⁴⁵ (0.74 pg/mg). However, higher concentrations of desipramine (10 μM) blocked ACh-induced CA secretion as well as Ca⁴⁵ accumulation over 90%. ³H-noradrenaline overflow evoked by sympathetic nerve stimulation in the perfused salivary gland was enhanced by 0.3 to 30 μM desipramine; higher concentration (100 μM) caused a substantial reduction. However, the reduction in overflow was mainly due to a marked increase in the spontaneous outflow of tritium by high concentrations (100 μM) of desipramine during the nonstimulation period. As high as 100 μM trifluoperazine did not inhibit ³H-noradrenaline release. However, these concentrations substantially increased the spontaneous outflow. With use of “calmodulin inhibitors” it was not possible to obtain unequivocal evidence for the participation of endogenous calmodulin in the secretion of CA or release of sympathetic transmitter. It is evident that these drugs can have multiple effects on the steps involved in the process of neurosecretion.