The rabbit aortic strip, guinea‐pig ileum and rabbit skeletal muscle sarcoplasmic reticulum preparations were used to determine at which sites and in what manner 8‐(N,N‐diethylamino)‐octyl 3,4,5‐trimethoxybenzoate (TMB‐8) interferes with Ca2+ availability in smooth and skeletal muscles. TMB‐8 (50 μM) significantly inhibited equivalent responses of the rabbit aortic strip to KCl and noradrenaline. TMB‐8 (65 μM) produced no significant alteration in the extracellular space of the guinea‐pig ileum as measured with [3 H]‐sorbitol. The resting cellular Ca2+ influx as well as the resting 45 Ca2+ efflux in the guinea‐pig ileum preparation were significantly inhibited by TMB‐8 (65 μM). TMB‐8 (5 μM and 50 μM) had no significant effect on the uptake of 45 Ca2+ by the sarcoplasmic reticulum preparation of skeletal muscle; however, TMB‐8 (5 μM) did significantly inhibit the caffeine (20 μ)‐induced release of 45 Ca2+ from this preparation. It is concluded that TMB‐8 reduces Ca2+ availability in smooth and skeletal muscles by stabilizing Ca2+ binding to cellular Ca2+ stores and thereby inhibits the release of this Ca2+ by contractile stimuli.

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