The present studies examined relationships between intraluminal pressure, membrane potential (E(m)), and myogenic tone in skeletal muscle arterioles. Using pharmacological interventions targeting Ca(2+) entry/release mechanisms, these studies also determined the role of Ca(2+) pathways and E(m) in determining steady-state myogenic constriction. Studies were conducted in isolated and cannulated arterioles under zero flow. Increasing intraluminal pressure (0-150 mmHg) resulted in progressive membrane depolarization (-55.3 +/- 4.1 to -29.4 +/- 0.7 mV) that exhibited a sigmoidal relationship between extent of myogenic constriction and E(m). Thus, despite further depolarization, at pressures >70 mmHg, little additional vasoconstriction occurred. This was not due to an inability of voltage-operated Ca(2+) channels to be activated as KCl (75 mM) evoked depolarization and vasoconstriction at 120 mmHg. Nifedipine (1 microM) and cyclopiazonic acid (30 microM) significantly attenuated established myogenic tone, whereas inhibition of inositol 1,4,5-trisphosphate-mediated Ca(2+) release/entry by 2-aminoethoxydiphenylborate (50 microM) had little effect. Combinations of the Ca(2+) entry blockers with the sarcoplasmic reticulum (SR) inhibitor caused a total loss of tone, suggesting that while depolarization-mediated Ca(2+) entry makes a significant contribution to myogenic tone, an interaction between Ca(2+) entry and SR Ca(2+) release is necessary for maintenance of myogenic constriction. In contrast, none of the agents, in combination or alone, altered E(m), demonstrating the downstream role of Ca(2+) mobilization relative to changes in E(m). Large-conductance Ca(2+)-activated K(+) channels modulated E(m) to exert a small effect on myogenic tone, and consistent with this, skeletal muscle arterioles appeared to show an inherently steep relationship between E(m) and extent of myogenic tone. Collectively, skeletal muscle arterioles exhibit complex relationships between E(m), Ca(2+) availability, and myogenic constriction that impact on the tissue's physiological function.

Ca2+ 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 Ca2+ (Ca2+i). 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 Ca2+i was measured using fluorescence imaging of Fura 2 loaded arterioles. Both nifedipine and mibefradil showed dose‐dependent inhibition of spontaneous myogenic tone (at 70 mmHg; pEC50 7.04±0.17 vs 6.65±0.20 respectively, n=6 for both, n.s.) and KCl‐induced vasoconstriction (at 70 mmHg; pEC50 6.93±0.38 vs 6.45±0.27 respectively, n=6 for both, n.s.). In arterioles maintained at 50 mmHg, nifedipine (10−7 and 10−5 M) caused a concentration dependent reduction in Ca2+i, however, mibefradil (10−7 and 10−5 M) had no effect. Furthermore nifedipine significantly attenuated the increase in Ca2+i associated with an acute pressure step (50–120 mmHg) whereas mibefradil was considerably less effective. Mibefradil (10−7 M) significantly attenuated contractile responses to 60 mM KCl without altering the KCl‐induced increase in Ca2+i, in contrast to nifedipine (10−7 M) which reduced both Ca2+i and contraction. 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. 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 Ca2+ levels suggest mibefradil exhibits actions other than blockade of Ca2+ entry in skeletal muscle arterioles. British Journal of Pharmacology (2000) 131, 1065–1072; doi:

As smooth muscle cell (SMC) membrane potential (E(m)) is critical for vascular responsiveness, and arteriolar SMCs are depolarized at physiological intraluminal pressures, we hypothesized that myogenic tone impacts on dilation mediated by endothelium-derived hyperpolarization (EDH). Studies were performed on cannulated mouse cremaster arterioles [diameter, 33+/-2 microm (n=23) at 60 mmHg; SMC Em -34.6+/-1.2 mV (n=7)]. Myogenic activity was assessed as tone developed in response to intraluminal pressure. Functional observations were related to mRNA, protein expression, and anatomy. Acetylcholine concentration-response curves showed a modest shift following indomethacin (10 microM) and L-NAME (100 microM), although maximal vasodilation was achieved. Residual dilation was removed by apamin (1 microM) in combination with TRAM-34 (1 microM) or charybotoxin (0.1 microM), indicating the requirement of small (S) and intermediate (I) calcium-activated potassium channels (K(Ca)). Charybdotoxin, but not TRAM-34, caused vasoconstriction, presumably through the inhibition of SMC BK(Ca). Expression of SK3 and IK1 was confirmed by immunohistochemistry and polymerase chain reaction, while myoendothelial junctions were common, suggesting a high degree of cell coupling. Also consistent with a role for endothelial K(Ca) channels, acetylcholine increased endothelium [Ca(2 +)](i). Apamin and TRAM-34 similarly blocked EDH-mediated dilation at intraluminal pressures of 30 and 90 mmHg, suggesting that in mouse arterioles, SK(Ca -) and IK(Ca -) mediated mechanisms predominate and operate independently of physiological levels of myogenic activation.

1 The actions of neuropeptide Y (NPY) and agonists selective for NPY receptor subtypes were examined on arterioles from the guinea-pig small intestine and the rat pia in order to characterize the receptors mediating the vasoconstrictor and potentiating effects of NPY. 2 A method was developed for measuring the potentiating effects of NPY in situations where it was not possible to obtain a full concentration-response relationship for the vasoconstrictor. NPY, 50 nM, had a greater potentiating effect on the guinea-pig intestinal arterioles than those from the rat pia.

to the dilatation. 6. In 48 % of arterioles tested the dilatation appeared to be mediated solely by acetylcholine. In 31 % there was a cholinergic component, but no evidence for the involvement of acetylcholine in the remaining 21 %. When the non-cholinergic dilatation occurred without a hyperpolarization we conclude that it was due to vasoactive intestinal peptide; otherwise it may have been due to either galanin or dynorphin.