Abstract
1. We have studied the effect of substance P (SP) on catecholamine (CA) secretion evoked by prolonged field stimulation from the perfused rat adrenal gland in vitro. 2. Previous studies show that SP has an inhibitory effect on the nicotinic response in a number of different tissue preparations. In the present study, we found that SP at concentrations from 10(-7) to 10(-6) M markedly increased CA secretion evoked by prolonged high-frequency field stimulation but not that evoked by low-frequency field stimulation. 3. In the absence of field stimulation SP by itself had no direct effect on CA secretion. This indicates that SP acts as a neuromodulator rather than a neurotransmitter to increase CA secretion from the adrenal medulla. 4. The nicotinic receptors were still functional following a period of prolonged field stimulation (120 min) because CA secretion was markedly increased in response to the nicotinic agonist dimethylphenylpiperazinium (DMPP). 5. Capsaicin pre-treatment reduced CA secretion evoked by prolonged field stimulation and the facilitatory action of SP on CA secretion lasted longer in these capsaicin-pre-treated rats than in controls, indicating that SP-containing capsaicin-sensitive fibres innervating the adrenal medulla may be involved in the regulation of CA secretion. 6. In parallel with the increase in CA secretion, 3H overflow from the splanchnic nerve pre-labelled with [3H]choline was also increased by SP. The increase in CA secretion by SP lasted longer than the increase in 3H overflow. These results suggest that SP facilitates CA secretion from the adrenal gland at two levels: (1) pre-synaptically by facilitating ACh release from splanchnic nerve terminals, and (2) post-synaptically by modulating the nicotinic secretory response by protection against nicotinic desensitization of secretion. 7. The present studies provide further evidence that endogenous SP in the splanchnic nerve may modulate CA secretion during stress.