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(Circulation Research. 1999;85:651-652.)
© 1999 American Heart Association, Inc.

Editorials

A Novel Ca²⁺ Channel in Vascular Smooth Muscle?

Jenafer Evans, Craig H. Gelband

From the Department of Physiology, University of Florida College of Medicine, Gainesville, Fla.

Correspondence to Dr Craig H. Gelband, Department of Physiology, University of Florida College of Medicine, PO Box 100274, Gainesville, FL 32610. E-mail Gelband@phys.med.ufl.edu

Key Words: arteriole • patch clamp • voltage-dependent Ca²⁺ channel • vascular tone

	Introduction

 
Contraction of vascular smooth muscle cells (VSMCs) requires an increase in [Ca²⁺]_i. This increase can occur via Ca²⁺ release from the sarcoplasmic reticulum or by influx of Ca²⁺ from the extracellular space through voltage-dependent Ca²⁺ channels (VDCCs) or receptor operated nonselective cation channels.¹ A number of studies using dissociated or cultured VSMCs with origins from a range of vascular beds have demonstrated, using electrophysiological and molecular techniques, that a major contribution to Ca²⁺ influx in response to membrane depolarization and/or vasoactive agents is mediated via VDCCs. Two types of VDCCs have been recorded in VSMCs: L- and T-type. The L-type Ca²⁺ current is classified as high voltage activated (HVA) because it activates at membrane potentials at or more positive than -40 mV. The L-type class of VDCCs has a distinct pharmacology. L-type VDCCs are dihydropyridine (DHP) sensitive, and molecular cloning has confirmed that these channels contain unique domains on the pore-forming subunit that confer this sensitivity. Because the L-type VDCC is a major contributor to vascular tone, its unique pharmacology has been exploited to treat cardiovascular disorders such as hypertension.¹ However, the ubiquitous expression of L-type Ca²⁺ channels in many cell types causes undesirable side effects when DHPs are clinically used.

Fewer published studies have shown that T-type Ca²⁺ current exists in smooth muscle cells.^{2 3 4 5 6 7} Unlike the HVA channels, electrophysiological data show that T-type Ca²⁺ current is DHP insensitive and activates at more negative membrane potentials. Additionally, T-type Ca²⁺ current inactivates whereas HVA channels show relatively little inactivation . . . [Full Text of this Article]