Inhibition of nicotinic receptor-mediated responses in bovine chromaffin cells by diltiazem

1 The effects of diltiazem on various functional parameters were studied in bovine cultured adrenal chromaffin cells stimulated with the nicotinic receptor agonist dimethylphenylpiperazinium (DMPP) or with depolarizing Krebs-HEPES solutions containing high K+ concentrations. 2 The release of [H-3]-noradrenaline induced by DMPP (100 mu M for 5 min) was gradually and fully inhibited by increasing concentrations of diltiazem (IC50 = 1.3 mu M). In contrast, the highest concentration of diltiazem used (10 mu M) inhibited the response to high K+ (59 mM for 5 min) by only 25%. 3 Ca-45(2+) uptake into cells stimulated with DMPP (100 mu M for 1 min) was also blocked by diltiazem in a concentration-dependent manner (IC50 = 0.4 mu M). Again, diltiazem blocked the K+-evoked Ca-45(2+) uptake (70 mM K+ for 1 min) only by 20%. In contrast, the N-P-Q-type Ca2+ channel blocker omega-conotoxin MVIIC depressed the K+ signal by 70%. In the presence of this toxin, diltiazem exhibited an additional small inhibitory effect, indicating that the compound was acting on L-type Ca2+ channels. 4 Whole-cell Ba2+ currents through Ca2+ channels in voltage-clamped chromaffin cells were inhibited by 3-10 mu M diltiazem by 20-25%. The inhibition was readily reversed upon washout of the drug. 5 The whole-cell currents elicited by 100 mu M DMPP (I-DMPP) were inhibited in a concentration-dependent and reversible manner by diltiazem. Maximal effects were found at 10 mu M, which reduced the peak I-DMPP, by 70%. The area of each curve represented by total current (Q(DMPP)) was reduced more than the peak current. At 10 mu M, the inhibition amounted to 80%; the IC50 for (Q(DMPP)) inhibition was 0.73 mu M, a figure close to the IC50 for Ca-45(2+) uptake (0.4 mu M) and [H-3]-noradrenaline release (1.3 mu M). The blocking effects of diltiazem developed very quickly and did not exhibit use-dependence; thus the drug blocked the channel in its closed state. The blocking effects of 1 mu M diltiazem on I-DMPP were similar at different holding potentials (inhibition by around 30% at -100, -80 or -50 mV). Diltiazem did not affect the current flow through voltage-dependent Na+ channels. 6 These data are compatible with the idea that diltiazem has little effect on Ca2+ entry through voltage-dependent Ca2+ channels in bovine chromaffin cells. Neither, does diltiazem affect I-Na. Rather, diltiazem acts directly on the neuronal nicotinic receptor ion channel and blocks ion fluxes, cell depolarization and the subsequent Ca2+ entry and catecholamine release. This novel effect of diltiazem might have clinical relevance since it might reduce the sympathoadrenal drive to the heart and blood vessels, thus contributing to the well established antihypertensive and cardioprotective effects of the drug.