Physiologic and pathophysiologic relevance of T-type calcium-ion channels: potential indications for T-type calcium antagonists.

The family of voltage-gated calcium-ion (Ca2+) channels is critical in the role of transmembrane signaling of excitable cells throughout the body. Within the cardiovascular system, two types of Ca2+ channels have been identified: the L-type channel and the T-type channel. These two types of Ca2+ channels have distinct electrophysiologic identities, and although the roles of the T-type Ca2+ channels have not been firmly established, there are many reasons for believing that the roles of the T-type and L-type Ca2+ channels are distinct. T-type Ca2+ channels have the appropriate characteristics to generate pacemaker activity in the sinoatrial node. In vascular smooth muscle, they appear to be involved in maintenance of coronary and peripheral vasomotor tone and control of vascular growth and remodeling. Characterization of the T-type Ca2+ channels will be facilitated by the availability of mibefradil, a novel calcium antagonist that selectively blocks T-type Ca2+ channels. Mibefradil is associated with a reduction in heart rate but not with negative inotropic effects or neurohormonal stimulation. It is thought that the unique pharmacologic effects of mibefradil are related to blockade of T-type Ca2+ channels, and it is hypothesized that this action will have a positive impact on cardiovascular morbidity and mortality via cardioprotective and renoprotective effects. However, much work needs to be done to fully test this hypothesis.