The quantal secretion of catecholamines is impaired by the accumulation of β‐adrenoceptor antagonists into chromaffin cell vesicles

Abstract

Background and purpose: The delayed onset of certain effects of antagonists of b-adrenoceptors (b-blockers), such as lowering arterial blood pressure (several days), cannot be explained solely by their effects on b-adrenoceptors, an action that occurs within minutes. Although several mechanisms have been proposed, none of them explain this temporal delay. This work aimed at providing a new explanation based on the interference of these drugs with the functional accumulation of catecholamines within neurosecretory vesicles. Experimental approach: We used the simultaneous on-line monitoring of catecholamine and labetalol release from bovine isolated chromaffin cells and from rat perfused adrenal glands, as well as single cell amperometry, intracellular electrochemistry, patch amperometry and HPLC. Key results: Using amperometry, three b-blockers, labetalol, atenolol and propranolol, reduced the quantal size of secretory events in chromaffin cells, accompanied by a slowing down of exocytosis. By patch amperometry, we found that treatment with b-blockers also increases the chromaffin vesicle volume, thereby creating a functional dilution of catecholamines. Experiments with intracellular electrochemistry show that vesicles cannot uptake new catecholamines. There was progressive accumulation of labetalol in secretory vesicles of bovine adrenal chromaffin cells, and this b-blocker was co-released with catecholamines from rat and bovine chromaffin tissues. Conclusions and implications:We propose that b-blockers are progressively concentrated into sympathetic secretory vesicles, and interfere with the storage of catecholamines and are co-released with the natural transmitters, resulting in a decrease in the sympathetic tone. This could explain the delayed onset of the hypotensive effects of b-blockers. (2010) Keywords: amperometry; chromaffin cells; patch amperometry; secretion; catecholamines; b-adrenoceptor; labetalol; propranolol; atenolol Abbreviations: b-blocker, b-adrenoceptor antagonist; CgB, chromogranin B; DMPP, dimethylphenyl piperazinium; Imax, maximal intensity of amperometrical spikes; m, ascending slope of spikes; Q, spike charge; t1/2, spike width at a half height British Journal of Pharmacology

Gabapentin Inhibits Catecholamine Release from Adrenal Chromaffin Cells

“…Hydralazine and some beta-blockers have been reported to accumulate in secretory vesicles of chromaffin cells and thereby displace catecholamines leading to reduced vesicular content and altered release kinetics 39,40 . Therefore, to assess these possibilities and gain more precise mechanistic insight we used carbon fiber amperometry.…”

“…Previous work demonstrated that second messenger pathways can alter the content and/or emptying of secretory vesicles upon stimulation 48–53 . Furthermore, hydralazine and β blockers can accumulate in secretory vesicles and thereby reduce the amount and kinetics of catecholamine release from each vesicle 39,40 . Using carbon fiber amperometry we show that gabapentin reduced the number of vesicles that undergo exocytosis, but did not change the amount or kinetics of catecholamine released from these unitary events (fig.…”

Fluorescent β-Blockers as Tools to Study Presynaptic Mechanisms of Neurosecretion

“…Recently, we also showed that the accumulation of β-Bs like atenolol, labetalol and propranolol (false neurotransmitters) has a significant effect on the quantum release of adrenaline [3]. Likewise, we demonstrated that the β-B labetalol was co-liberated with catecholamines.…”

Chemical Analysis of Single Cells

“…104 Similarly, it has been reported that beta-blockers can accumulate into the vesicles of bovine chromaffin cells, thus decreasing the concentration of released catecholamines and inducing a decrease in sympathetic tone. 105 This result could explain, in part, the delay in the hypotensive effect of beta-blockers.…”