Specific binding of the Ca++ antagonist desmethoxyverapamil, (-)-[3H]D888, to cell membranes of equine portal vein smooth muscle was inhibited in a concentration-dependent manner by guanosine 5'-O-(gamma-thio)triphosphate and ATP but was little affected by guanosine 5'-O-(beta-thio)diphosphate, noradrenaline or phorbol 12-myristate 13-acetate ester. Inhibition constants for GTP and ATP were in the range of 0.1 to 0.3 mM. From Scatchard plots and dissociation kinetic experiments, it is proposed that D888 high affinity binding sites are transferred into low affinity sites. In intact strips of rat portal vein bathed in physiological solution, both noradrenalin and a combination of aluminum chloride and sodium fluoride inhibited (-)-[3H]D888 binding, whereas guanosine 5'-O-(gamma-thio)triphosphate was without effect. In strips pretreated with 1 microM prazosin or 10 micrograms/ml pertussis toxin (PTX) for 6 h, noradrenalin had no effect on specific (-)-[3H]D888 binding. In addition, inhibition of (-)-[3H]D888 binding in the presence of 3 microM noradrenalin was reversed in a concentration-dependent manner by prazosin but not by propranolol. Noradrenalin-induced contractions were inhibited in a concentration-dependent manner by D888. In strips preincubated with 10 micrograms/ml PTX for 6 h, the concentration-response curve was shifted to the left, indicating that removal of the PTX sensitive transduction pathway increased D888 affinity for its specific binding sites. These results show that (-)-[3H]D888 binding to Ca++ channels is changed by GTP analogs in a way that suggests that a PTX-sensitive guanine nucleotide-binding protein may directly interact with Ca++ channels in response to activation of alpha 1 adrenoceptors.