Studies on secretion of catecholamines evoked by acetylcholine or transmural stimulation of the rat adrenal gland.

Abstract

A method of studying the secretion of catecholamines (CA) in the isolated perfused rat adrenal gland by transmural stimulation or by application of acetylcholine (ACh) is described. Secretion of CA was practically linear in response to ACh administration, starting from 4.42 .mu.M to 1.32 mM. Transmural stimulation enhanced secretion from a stimulation frequency of 0.5-3 Hz; the effect leveled at 10 Hz, and declined as frequency was raised to 30 Hz. The secretory response to transmural stimulation was maximal over 1 ms duration and 60 V. Secretion evoked by transmural stimulation was blocked (70-95%) by 0.31 .mu.M tetrodotoxin (TTX) irrespective of stimulus duration, voltage and frequency of stimulation. Secretion evoked by ACh was depressed 43% by TTX. After mecamylamine (0.59 mM) treatment, secretory response evoked by either procedure was blocked by about 80%. Adenosine (0.18 mM), AMP (0.28 mM), or ATP (0.19 mM) lowered CA secretion evoked by transmural stimulation by about 40%, but had no effect on secretion induced by ACh. Isoprenaline (4.52 .mu.M), propranolol (11.58 .mu.M), clonidine (13.00 .mu.M), phenoxybenzamine (3.30 .mu.M), and 4-aminopyridine (3 mM) did not modify CA secretion evoked by transmural stimulation or by ACh. Perfusion of the adrenal gland with 0.25 mM Ca-Krebs solution completely abolished CA secretion evoked by transmural stimulation, but ACh-induced secretion was still 30-50% of the control value; 20 mM Mg blocked electrically induced secretion by 60%, but that evoked by ACh was unaffected. Perfusion with Ca-free Krebs solution for 2 h did not completely abolish the response. Treatment with EGTA [ethylene glycol-bis(.beta.-amino ethyl ether) N,N,N'',N''-tetraacetic acid] (5 mM) for 30 min totally blocked ACh-induced secretion. La or Mn were more effective in blocking transmurally evoked secretion than ACh-evoked secretion of CA. Verapamil (0.1 mM) had no significant effect on secretion evoked by either procedure. A 5-fold increase in its concentration caused about 75% blockade of secretion. Differential effects of various ions and agents on CA secretion are explained on the basis that these compounds affect neurosecretory properties of the presynaptic splanchnic nerve terminals and of chromaffin cells differently.