Inhibition of myogenic tone by mibefradil in rat cerebral arteries

T-type Ca2+ channels and autoregulation of local blood flow

L-type voltage gated Ca²⁺ channels are considered to be the primary source of calcium influx during the myogenic response. However, many vascular beds also express T-type voltage gated Ca²⁺ channels. Recent studies suggest that these channels may also play a role in autoregulation. At low pressures (40-80 mmHg) T-type channels affect myogenic responses in cerebral and mesenteric vascular beds. T-type channels also seem to be involved in skeletal muscle autoregulation. This review discusses the expression and role of T-type voltage gated Ca²⁺ channels in the autoregulation of several different vascular beds. Lack of specific pharmacological inhibitors has been a huge challenge in the field. Now the research has been strengthened by genetically modified models such as mice lacking expression of T-type voltage gated Ca²⁺ channels (Ca_V3.1 and Ca_V3.2). Hopefully, these new tools will help further elucidate the role of voltage gated T-type Ca²⁺ channels in autoregulation and vascular function.

Evidence both L-type and non-L-type voltage-dependent calcium channels contribute to cerebral artery vasospasm following loss of NO in the rat

We recently found block of NO synthase in rat middle cerebral artery caused spasm, associated with depolarizing oscillations in membrane potential (E_m) similar in form but faster in frequency (circa 1 Hz) to vasomotion. T-type voltage-gated Ca²⁺ channels contribute to cerebral myogenic tone and vasomotion, so we investigated the significance of T-type and other ion channels for membrane potential oscillations underlying arterial spasm. Smooth muscle cell membrane potential (E_m) and tension were measured simultaneously in rat middle cerebral artery. NO synthase blockade caused temporally coupled depolarizing oscillations in cerebrovascular E_m with associated vasoconstriction. Both events were accentuated by block of smooth muscle BK_Ca. Block of T-type channels or inhibition of Na⁺/K⁺-ATPase abolished the oscillations in E_m and reduced vasoconstriction. Oscillations in E_m were either attenuated or accentuated by reducing [Ca²⁺]_o or block of K_V, respectively. TRAM-34 attenuated oscillations in both E_m and tone, apparently independent of effects against K_Ca3.1. Thus, rapid depolarizing oscillations in E_m and tone observed after endothelial function has been disrupted reflect input from T-type calcium channels in addition to L-type channels, while other depolarizing currents appear to be unimportant. These data suggest that combined block of T and L-type channels may represent an effective approach to reverse cerebral vasospasm.

State-dependent verapamil block of the cloned human Ca(v)3.1 T-type Ca(2+) channel

Verapamil is a potent phenylalkylamine antihypertensive believed to exert its therapeutic effect primarily by blocking high-voltage-activated L-type calcium channels. It was the first clinically used calcium channel blocker and remains in clinical use, although it has been eclipsed by other calcium channel blockers because of its short half-life and interactions with other channels. In addition to blocking L-type channels, it has been reported to block T-type (low-voltage activated) calcium channels. This type of cross-reactivity is likely to be beneficial in the effective control of blood pressure. Although the interactions of T channels with a number of drugs have been described, the mechanisms by which these agents modulate channel activity are largely unknown. Most calcium channel blockers exhibit state-dependence (i.e., preferential binding to certain channel conformations), but little is known about state-dependent verapamil block of T channels. We stably expressed human Ca(v)3.1 T-type channels in human embryonic kidney 293 cells and studied the state-dependence of the drug with macroscopic and gating currents. Verapamil blocked currents at micromolar concentrations at polarized potentials similar to those reported for L-type channels, although unlike for L-type currents, it did not affect current time course. The drug exhibited use-dependence and significantly slowed the apparent recovery from inactivation. Current inhibition was dependent on potential. This dependence was restricted to negative potentials, although all data were consistent with verapamil binding in the pore. Gating currents were unaffected by verapamil. We propose that verapamil achieves its inhibitory effect via occlusion of the channel pore associated with an open/inactivated conformation of the channel.

Antihypertensive Effects of the Putative T-Type Calcium Channel Antagonist Mibefradil Are Mediated by the L-Type Calcium Channel Ca v 1.2

The role of T-type Ca 2+ channels for cardiovascular physiology, in particular blood pressure regulation, is controversial. Selective blockade of T-type Ca 2+ channels in resistance arteries has been proposed to explain the effect of the antihypertensive drug mibefradil. In the present study, we used a third generation, time- and tissue-specific conditional knockout model of the L-type Ca 2+ channel Ca v 1.2 (Ca v 1.2 SMAKO mice) to genetically dissect the effects of mibefradil on T- and L-type Ca 2+ channels. Myogenic tone and phenylephrine-induced contraction in hindlimb perfusion experiments were sensitive to mibefradil in control mice, whereas the drug showed no effect in Ca v 1.2-deficient animals. Mean arterial blood pressure in awake, freely moving control mice was reduced by 38±2.5 mm Hg at a dose of 1.25 mg/kg bodyweight mibefradil, but not changed in Ca v 1.2 SMAKO mice. These results demonstrate that the effect of the putative T-type Ca 2+ channel-selective blocker mibefradil on blood pressure and small vessel myogenic tone is mediated by the Ca v 1.2 L-type Ca 2+ channel.

Role of T-type calcium channels in myogenic tone of skeletal muscle resistance arteries

T-type calcium channels may be involved in the maintenance of myogenic tone. We tested their role in isolated rat cremaster arterioles obtained after CO(2) anesthesia and decapitation. Total RNA was analyzed by RT-PCR and Southern blotting for calcium channel expression. We observed expression of voltage-operated calcium (Ca(V)) channels Ca(V)3.1 (T-type), Ca(V)3.2 (T-type), and Ca(V)1.2 (L-type) in cremaster arterioles (n = 3 rats). Amplification products were observed only in the presence of reverse transcriptase and cDNA. Concentration-response curves of the relatively specific L-type blocker verapamil and the relatively specific T-type blockers mibefradil and nickel were made on cannulated vessels with either myogenic tone (75 mmHg) or a similar level of constriction induced by 30 mM K(+) at 35 mmHg. Mibefradil and nickel were, respectively, 162-fold and 300-fold more potent in inhibiting myogenic tone compared with K(+)-induced constriction [log(IC(50), M): mibefradil, basal -7.3 +/- 0.2 (n = 9) and K(+) -5.1 +/- 0.1 (n = 5); nickel, basal -4.1 +/- 0.2 (n = 5) and K(+) -1.6 +/- 0.5 (n = 5); means +/- SE]. Verapamil had a 17-fold more potent effect [log(IC(50), M): basal -6.6 +/- 0.1 (n = 5); K(+) -5.4 +/- 0.3 (n = 4); all log(IC(50)) P < 0.05, basal vs. K(+)]. These data suggest that T-type calcium channels are expressed and involved in maintenance of myogenic tone in rat cremaster muscle arterioles.

Invited review: mechanisms of calcium handling in smooth muscles

The concentration of cytoplasmic Ca(2+) regulates the contractile state of smooth muscle cells and tissues. Elevations in global cytoplasmic Ca(2+) resulting in contraction are accomplished by Ca(2+) entry and release from intracellular stores. Pathways for Ca(2+) entry include dihydropyridine-sensitive and -insensitive Ca(2+) channels and receptor and store-operated nonselective channels permeable to Ca(2+). Intracellular release from the sarcoplasmic reticulum (SR) is accomplished by ryanodine and inositol trisphosphate receptors. The impact of Ca(2+) entry and release on cytoplasmic concentration is modulated by Ca(2+) reuptake into the SR, uptake into mitochondria, and extrusion into the extracellular solution. Highly localized Ca(2+) transients (i.e., sparks and puffs) regulate ionic conductances in the plasma membrane, which can provide feedback to cell excitability and affect Ca(2+) entry. This short review describes the major transport mechanisms and compartments that are utilized for Ca(2+) handling in smooth muscles.

Effects of mibefradil and nifedipine on arteriolar myogenic responsiveness and intracellular Ca(2+)

1. Ca(2+) entry mechanisms underlying spontaneous arteriolar tone and acute myogenic reactivity remain uncertain. These studies aimed to compare the effects of nifedipine and the putative T-channel blocker, mibefradil, on arteriolar myogenic responsiveness and intracellular Ca(2+) (Ca(2+)(i)). 2. First order cremaster muscle arterioles (1A) were isolated from rats, cannulated, pressurized to 70 mmHg in the absence of intraluminal flow, and mechanical responses studied by video microscopy. The Ca(2+)(i) was measured using fluorescence imaging of Fura 2 loaded arterioles. 3. Both nifedipine and mibefradil showed dose-dependent inhibition of spontaneous myogenic tone (at 70 mmHg; pEC(50) 7.04+/-0.17 vs 6.65+/-0.20 respectively, n=6 for both, n.s.) and KCl-induced vasoconstriction (at 70 mmHg; pEC(50) 6.93+/-0. 38 vs 6.45+/-0.27 respectively, n=6 for both, n.s.). 4. In arterioles maintained at 50 mmHg, nifedipine (10(-7) and 10(-5) M) caused a concentration dependent reduction in Ca(2+)(i), however, mibefradil (10(-7) and 10(-5) M) had no effect. Furthermore nifedipine significantly attenuated the increase in Ca(2+)(i) associated with an acute pressure step (50 - 120 mmHg) whereas mibefradil was considerably less effective. 5. Mibefradil (10(-7) M) significantly attenuated contractile responses to 60 mM KCl without altering the KCl-induced increase in Ca(2+)(i), in contrast to nifedipine (10(-7) M) which reduced both Ca(2+)(i) and contraction. 6. Membrane potential of arterioles with spontaneous myogenic tone (70 mmHg) was -41.5+/-1. 0 mV. Nifedipine (10(-7) or 10(-5) M) had no effect on membrane potential, however mibefradil (10(-5) M) caused significant depolarization. 7. In summary, both mibefradil and nifedipine inhibit arteriolar spontaneous tone and acute myogenic reactivity. While there may be overlap in the mechanisms by which these agents inhibit tone, differences in effects on membrane potential and intracellular Ca(2+) levels suggest mibefradil exhibits actions other than blockade of Ca(2+) entry in skeletal muscle arterioles.

Influence of T-type Ca2+ (mibefradil) and Cl- (indanyloxyacetic acid 94) channel antagonists on α1-adrenoceptor mediated contractions in rat aorta

The effects of the T-type and L-type Ca2+ channel antagonists, mibefradil and nifedipine, respectively, and those of a Cl- channel antagonist, indanyloxyacetic acid 94, on mechanical responses elicited by selective activation of α1-adrenoceptors using cirazoline were examined in rat isolated aortic rings. The presence of mibefradil (300 nM), indanyloxyacetic acid, 94 (30 µM) and nifedipine (300 nM) alone inhibited mechanical responses elicited by cirazoline. The concentration-response curves to cirazoline were displaced to the right with significant increases in the EC50 and significant depressions of the maximal responses in the presence of the individual agents mibefradil, indanyloxyacetic acid 94, or nifedipine. A combination of mibefradil and indanyloxyacetic acid 94 further inhibited the mechanical activity produced by cirazoline. The further reduction in the maximal response to cirazoline, in the presence of mibefradil and nifedipine, was insignificant when compared with the effects of nifedipine alone. In addition, maximal mechanical responses produced by cirazoline were not significantly affected by a combination of nifedipine and indanyloxyacetic acid 94 when compared with either nifedipine alone or mibefradil and indanyloxyacetic acid 94 combined. Our current findings indicate that mibefradil, indanyloxyacetic acid 94, and nifedipine can inhibit cirazoline-induced contractions to a varying degree. Moreover, based on our present data it would be reasonable to suggest that the contribution of T-type versus L-type Ca2+ channels to contractile responses obtained with cirazoline are approximately 21% and 35%, respectively, of the Emax. It would appear that L-type Ca2+ channels play a greater role in processes that are involved in excitation-contraction coupling subsequent to stimulation of α1-adrenoceptors. In addition, Cl- channels also appear to be involved in the process of contraction following α1-adrenoceptor activation.Key words: T-type Ca2+ channels, L-type Ca2+ channels, Cl- channels, isolated aortic rings.