Comparative study of d-diltiazem and its analogues as calcium antagonists in snail neurones

The blocking effects of d-diltiazem, its metabolites, deacetyl-d-diltiazem (d-M1), deacetyl-N-demethyl-d-diltiazem (d-M2), deacetyl-O-demethyl-d-diltiazem (d-M4) and deacetyl-N, O-demethyl-d-diltiazem (d-M6) and 1-diltiazem were investigated on the voltage-gated calcium current (ICa) of an Achatina neurone. Based on the IC50 values, the order of potency was: d-diltiazem (0.426 mM), d-M2 (0.456), d-M1 (0.491), 1-diltiazem (0.759) greater than d-M4 (1.212) greater than d-M6 (greater than 2.000). Assuming that the IC50 reflects the KD for resting Ca2+ channels (Kr), steady state activation studies gave KD values for the inactivated channels (Ki) and Kr/Ki ratios of 0.122 mM (Ki) and 3.52 (Kr/Ki) (d-diltiazem), 0.112 and 6.98 (1-diltiazem), 0.083 and 6.07 (d-M1) and 0.156 and 2.97 (d-M2). All drugs tested showed a certain degree of voltage-dependence. The further percentage reduction in the normalized ICa after high frequency stimulation demonstrated the use-dependence of: 1-diltiazem (27.5%), d-diltiazem (26.3) greater than d-M2 (19.2) greater than d-M1 (16.7) greater than d-M6 (9.8). The voltage- and use-dependence of these drugs are consistent with their Ca2+ antagonistic properties.