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

Abstract

The blockade of exocytosis induced by the putative endogenous ligand for imidazoline receptors, agmatine, was studied by using on-line measurement of catecholamine release in bovine adrenal medullary chromaffin cells. Agmatine inhibited the acetylcholine-evoked release of catecholamines in a concentration-dependent manner (IC 50=366 μM); the K +-evoked release of catecholamines was unaffected. Clonidine (100 μM) and moxonidine (100 μM) also inhibited by 75% and 50%, respectively, the acetylcholine-evoked response. In cells voltage-clamped at −80 mV, the intermittent application of acetylcholine pulses elicited whole-cell inward currents ( I ACh) that were blocked 63% by 1 mM agmatine. The onset of blockade was very fast ( τ on=31 ms); the recovery of the current after washout of agmatine also occurred very rapidly ( τ off=39 ms). Efaroxan (10 μM) did not affect the inhibition of I ACh elicited by 1 mM agmatine. I ACh was blocked 90% by 100 μM clonidine and 50% by 100 μM moxonidine. The concentration–response curve for acetylcholine to elicit inward currents was shifted to the right in a non-parallel manner by 300 μM agmatine. The blockade of I ACh caused by agmatine (100 μM) was similar at various holding potentials, around 50%. When intracellularly applied, agmatine did not block I ACh. At 1 mM, agmatine blocked I Na by 23%, I Ba by 14%, I K(Ca) by 16%, and I K(VD) by 18%. In conclusion, agmatine blocks exocytosis in chromaffin cells by blocking nicotinic acetylcholine receptor currents. In contrast to previous views, these effects seem to be unrelated to imidazoline receptors.