Blockade by agmatine of catecholamine release from chromaffin cells is unrelated to imidazoline receptors

The blockade of exocytosis induced by the putative endogenous ligand for im idazoline receptors, agmatine, was studied by using on-line measurement of catecholamine release in bovine adrenal medullary chromaffin cells. Agmatin e inhibited the acetylcholine-evoked release of catecholamines in a concent ration-dependent manner (IC50 = 366 muM); the K+-evoked release of catechol amines was unaffected. Clonidine (100 muM) and moxonidine (100 muM) also in hibited by 75% and 50%, respectively, the acetylcholine-evoked response. In cells voltage-clamped at -80 mV, the intermittent application of acetylcho line pulses elicited whole-cell inward currents (I-ACh) that were blocked 6 3% by 1 mM agmatine. The onset of blockade was very fast (tau (on) = 31 ms) ; the recovery of the current after washout of agmatine also occurred very rapidly (tau (off) = 39 ms). Efaroxan (10 muM) did not affect the inhibitio n of I-ACh elicited by 1 mM agmatine. I-ACh was blocked 90% by 100 muM clon idine and 50% by 100 muM moxonidine. The concentration-response curve for a cetylcholine to elicit inward currents was shifted to the right in a non-pa rallel manner by 300 muM agmatine. The blockade of I-ACh caused by agmatine (100 muM) was similar at various holding potentials, around 50%. When intr acellularly applied, agmatine did not block I-ACh. At 1 mM, agmatine blocke d I-Na by 23%, I-Ba by 14%, I-K(Ca) by 16%, and I-K(VD) by 18%. In conclusi on, agmatine blocks exocytosis in chromaffin cells by blocking nicotinic ac etylcholine receptor currents. In contrast to previous views, these effects seem to be unrelated to imidazoline receptors. (C) 2001 Elsevier Science B .V. All rights reserved.