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(Circulation Research. 2001;89:560.)
© 2001 American Heart Association, Inc.

Editorial

Vascular Smooth Muscle Calcium Channels

Could "T" Be a Target?

Leanne L. Cribbs

From the Cardiovascular Institute, Loyola University Medical Center, Maywood, Ill.

Correspondence to Leanne L. Cribbs, Cardiovascular Institute, Loyola University Medical Center, 2160 S First Ave, Maywood, IL 60153. E-mail lcribbs{at}lumc.edu

Key Words: calcium • channel • kidney • T-type

Voltage-gated Ca²⁺ channels of the plasma membrane open in response to changes in cell membrane potential, providing an important pathway for regulated entry of extracellular Ca²⁺ ions. Ca²⁺ channels comprise a family of structurally related proteins displaying tissue- and stage-specific expression. They are classified as either high voltage-activated (HVA) or low voltage-activated (LVA) and can be subdivided further based on their pharmacological properties. HVA channels include L-, N-, P-/Q-, and R-types, and LVA channels are designated as T-type. Members of each subtype have been identified by molecular cloning, facilitating studies of their biophysical properties, relative distribution, and contribution to Ca²⁺-dependent cellular functions. In recent years, it has become increasingly clear that in most, if not all, cases, more than one class of voltage-dependent Ca²⁺ channel are expressed in a given cell type. The functional relevance of different Ca²⁺ channels within a cell, however, remains unclear.

Ca²⁺ Channels and VSM Tone

In contractile cells, voltage-gated Ca²⁺ channels are particularly important because influx of extracellular Ca²⁺ is essential for muscle contraction and maintenance of tension. Ca²⁺ channels have received a great deal of attention in vascular smooth muscle (VSM), where calcium influx affects arterial vasoconstriction and vasorelaxation, and ultimately influences systemic blood pressure. Resistance arteries are an important site of blood pressure regulation. These arteries normally exist in a relatively contracted state and can undergo either dilation or constriction, depending on the contractile state of the arterial VSM. Ca²⁺ channel activity is a key determinant of VSM contractile state, and L-type Ca²⁺ channels are the classic target of calcium channel blockers (CCBs) widely prescribed as antihypertensive drugs. Therefore, a wealth of information has been published comparing different calcium antagonists, or CCBs, and their inhibitory affects on L-type calcium currents and vasoconstriction.

It is important to note that L-type Ca²⁺ channels are not the only class of voltage-gated Ca²⁺ channel in VSM, which is an important feature of the study by Hansen et al¹ in this issue of Circulation Research. Both HVA and LVA Ca²⁺ currents have been observed previously in VSM cells, for example, from aorta, mesenteric artery, coronary artery, rabbit ear artery, and renal arteries.^2–6 However, functional information relating T-type Ca²⁺ channels to VSM contractile state is scant. If the relative importance of T-type Ca²⁺ channels in VSM could be better defined, then this class of Ca²⁺ channels might become a viable target for an alternative class of CCB antihypertensives.

Multiple Ca²⁺ Channels in Renal Arteries

The study by Hansen et al¹ extends our knowledge about the Ca²⁺ channel constituents of VSM, specifically in renal resistance arteries. Both molecular and functional data support the idea that in addition to dihydropyridine-sensitive L-type channels, T-type channels also contribute to maintenance of vascular tone in the renal vasculature. The regional distribution of different Ca²⁺ channel isotypes is investigated by microdissection of pre- and postglomerular vessels from rat kidney, followed by reverse transcriptase–polymerase chain reaction specific for Ca²⁺ channel -subunits. Afferent arterioles isolated from rabbit kidney are used for functional studies using CCBs selective for L- versus T-type channels. Microdissected vessels show K⁺-induced responses in intracellular calcium, and isolated perfused arterioles display K⁺ depolarization-induced vasoconstriction. Both responses are inhibited by either calciseptine or mibefradil, indicating that in this model, both L-type and T-type Ca²⁺ channels are necessary for the vasoconstrictor response.

Previously, both L- and T-type Ca²⁺ currents were measured in VSM cells from renal afferent arterioles,⁶ but the molecular identity of channels producing these currents was not known. Hansen et al¹ report expression of Ca_v1.2 (HVA, L-type) along with LVA, T-type Ca²⁺ channels Ca_v3.1 and Ca_v3.2 in afferent and juxtamedullary efferent arterioles of rat kidney. This diversity of -subunits is consistent with previous molecular data showing multiple Ca²⁺ channels expressed in kidney, belonging to both the HVA^7,8 and LVA classes.^9–11 The present authors are the first to localize molecular expression of specific channel types to dissected pre- and postglomerular arterioles. In another recent publication, the same authors also detected mRNA and protein for the P-/Q-type channel (Ca_v2.1) in kidney, specifically in VSM cells from renal afferent arterioles.¹² In a similar series of experiments, incubation of isolated vessels with -Agatoxin IVA (a specific P-type Ca²⁺ channel blocker) completely abolished K⁺-induced vasoconstriction.

While it is evident that multiple voltage-gated Ca²⁺ channel types are present in renal resistance arteries, it is also intriguing that blocking any single class can have profound effects on depolarization-induced vasoconstriction, at least in the isolated arteriole model. It therefore appears that in addition to L-type Ca²⁺ channels, P-/Q-type, and now T-type, Ca²⁺ channels contribute to arterial VSM tone, albeit by unknown mechanisms.

L-Type Versus T-Type Channels in VSM

Arterial smooth muscle tone can be regulated by a diverse group of vasodilators, but it is primarily controlled by membrane potential through the activity of voltage-gated Ca²⁺ channels.¹³ In the kidney, voltage-gated Ca²⁺ channels mediate afferent autoregulatory responses to changes in pressure, which are believed to provide the primary protection against hypertensive glomerular injury. The beneficial effects of L-type calcium antagonists on renal hemodynamics and blood pressure have been studied extensively, as reviewed by Epstein and Loutzenhiser.¹⁴ The characterization of LVA, T-type Ca²⁺ channels in renal resistance arteries is significant, because more recent work suggests that preferential block of T-type channels with mibefradil lowers blood pressure and improves renal hemodynamics^15,16 (but see Griffin et al¹⁷). Unfortunately, direct experimental data on LVA Ca²⁺ channels in VSM are sparse, owing to the relative difficulty of measuring currents from VSM cell preparations, combined with the lack of highly specific T-type Ca²⁺ channel blockers. Functional studies, including those in this issue of Circulation Research, are limited by the use of agents with relatively low selectivity over L-type channels, such as mibefradil or low micromolar concentrations of Ni²⁺.

The kinetic properties of T-type Ca²⁺ channels are notably different from L-type channels, making it likely that they have a specialized function in VSM. Their low voltage range of activation makes them active at pacemaker potentials, and their involvement in cardiac pacemaking is well-established.^18,19 Branching points along resistance arteries may be an important site for propagation of spontaneous calcium oscillations.^20,21 L-type Ca²⁺ channels were previously localized in branch points of renal resistance arteries, where they contribute to calcium responses to membrane depolarization.²² Immunological stain for L-type Ca²⁺ channels in the study by Hansen et al¹ shows even staining across renal arterioles, and immunological data for T-type Ca²⁺ channels are not provided. Still, it remains plausible that the T-type Ca²⁺ channels could also participate in the propagation of calcium transients in VSM and the initiation of vasoconstriction.

Regional Vascular Heterogeneity: Afferent Versus Efferent

A discussion of vascular heterogeneity is warranted in light of the interesting results of Hansen et al.¹ Microheterogeneity of vascular beds is perhaps best exemplified by the renal microcirculation. The relative effect of calcium antagonists on preglomerular (afferent) versus postglomerular (efferent) arterioles continues to be controversial (eg, see Epstein and Loutzenhiser,¹⁴ Helou and Marchetti,²³ and the present study¹). Although a voltage-gated Ca²⁺ channel-dependent mechanism for calcium influx is generally accepted for afferent arterioles, a more prominent role for store-operated Ca²⁺ channels in rat efferent arterioles has recently been proposed.²⁴ Hansen et al¹ now present evidence that multiple Ca²⁺ channels are present not only in afferent vessels but also in juxtamedullary efferent arterioles, which also displayed depolarization-dependent calcium influx. Consistent with reported unresponsiveness of efferent arterioles to CCBs,^25,26 no Ca²⁺ channel -subunits were detected in cortical efferent arterioles. These data indicate that functional (and molecular) heterogeneity is present even among efferent renal arterioles, depending on relative proximity to the glomerular apparatus.

The diversity of Ca²⁺ channel types and functional response seen in the renal vasculature alone suggests that mechanisms of Ca²⁺ regulation may vary considerably in VSM, and generalizations should not be made. A combination of molecular and functional approaches such as presented here will be necessary for each preparation to define components of calcium handling, which may be relevant for the design of new, more effective therapeutic agents.

Future Considerations

It is difficult to unequivocally assign molecular correlates of functional effects observed in rabbit afferent arterioles used by Hansen et al.¹ Direct current measurements were not done in cell preparations from renal arterioles, so information about the relative Ca²⁺ channel current densities for the different channels is not available. It would also be desirable to correlate pharmacological block of particular current types with the observed functional block of K⁺-induced vasoconstriction by the same agents. If this were possible in the case of Ni²⁺ block, relative contribution of Ca_v3.1 versus Ca_v3.2 might be better defined.

A major technical issue, also pointed out by the authors, is that species differences in compartmental (ie, afferent versus efferent) Ca²⁺ channel subtype expression were not ruled out in this study. Rather, inability to detect any Ca²⁺ channel ₁-subunits in RNA from rabbit juxtamedullary efferents was in contrast to results in similar efferent arterioles from rat, where all three -subunits found in afferent arterioles (Ca_v1.2, Ca_v3.1, and Ca_v3.2) were, in fact, detected by PCR amplification. Furthermore, functional studies are done on rabbit afferent arterioles, and correlation of Ca²⁺ channel subtype with function makes the assumption that the constituent channels are the same in rat and rabbit, and that renal function does not differ greatly between species.

Hansen et al¹ present results on the renal microvasculature that raise intriguing questions about Ca²⁺ channel diversity in VSM. However, many more experiments are needed to tease out the importance of multiple Ca²⁺ channels in VSM cells and to elucidate a potential role for T-type channels. As we gain a better understanding of the molecular targets for beneficial effects of CCBs, advancement of knowledge about the contribution of LVA Ca²⁺ channels will rely on the development of much-needed specific T-type channel blockers and perhaps the generation of genetic overexpression or loss-of-function models for particular Ca²⁺ channel -subunit genes.

Footnotes

The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.

References

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