Physiological pharmacokinetics and pharmacodynamics of (±)‐verapamil in female rats

Tissue distribution and pharmacodynamics of verapamil were evaluated during steady state intravenous (i.v.) infusion and after single dose intraperitoneal (i.p.) drug administration to female Sprague–Dawley rats. In one group of rats, verapamil was infused to a steady state concentration at which time animals were killed. Verapamil‐induced decreases in mean arterial pressure (MAP) were monitored during infusion and correlated with concomitantly obtained plasma verapamil concentrations. Tissue (lung, liver, renal medulla, renal cortex, cardiac muscle, skeletal muscle, perirenal fat, brain stem, cerebral cortex, and cerebellum) and plasma samples were obtained immediately after animals were killed and verapamil and norverapamil concentrations determined. Another group of rats, after receiving i.p. verapamil, were killed at 1, 3, 5, 19, and 24 h. Elimination from each tissue evaluated was described by a first order process. Elimination half‐life of verapamil was similar among plasma and tissues evaluated (1.5 to 2.2 h). The per cent verapamil not bound to plasma proteins was concentration‐independent and similar between rats recieving i.p. (mean ± S.D.) (2.28 ± 0.72 per cent) and i.v. (2.08 ± 0.03 per cent) verapamil. MAP and verapamil concentration in plasma (r = 0.75; p < 0.01) and cardiac muscle (r = .0.82; p < 0.01) were inversely correlated in a highly significant fashion during both i.v. and i.p. drug administrations. The tissue‐to‐plasma distribution ratio for verapamil and norverapamil was similar among animals receiving i.p. verapamil at all points of sampling, suggesting distribution equilibrium had been achieved. After steady state i.v. infusion, both verapamil and norverapamil tissue: plasma concentration ratios were greater than after i.p. administration. Higher tissue: plasma verapamil concentration ratios after i.v. administration than after i.p. administration suggest either only a pseudoequilibrium is attained after i.p. administration or that determinants of tissue distribution of racemic verapamil differ with different routes of drug administration. In these studies, MAP provided a reasonable pharmacodynamic marker for verapamil tissue and plasma concentrations.