D600 blocks the Ca2+ channel from the outer surface of smooth muscle cell membrane of the rabbit intestine and portal vein.

The voltage dependent Ca2+ inward current in single smooth muscle cells dispersed from the longitudinal muscle layer of the rabbit ileum and rabbit portal vein was recorded using the whole-cell voltage clamp technique. D600 added to the bathing solution inhibited the Ca2+ current, while the intracellular perfusion of this agent did not reduce the amplitude of this current. Thus, D600 probably acts from the outer surface of the membrane. The nature of the Ca2+ channel in smooth muscle cells seems to differ from that in cardiac muscle cells.