Hypoxia inhibits contraction but not calcium channel currents or changes in intracellular calcium in arteriolar muscle cells

Voltage-dependent inward currents in smooth muscle cells of skeletal muscle arterioles

Voltage-dependent inward currents responsible for the depolarizing phase of action potentials were characterized in smooth muscle cells of 4^th order arterioles in mouse skeletal muscle. Currents through L-type Ca²⁺ channels were expected to be dominant; however, action potentials were not eliminated in nominally Ca²⁺-free bathing solution or by addition of L-type Ca²⁺ channel blocker nifedipine (10 μM). Instead, Na⁺ channel blocker tetrodotoxin (TTX, 1 μM) reduced the maximal velocity of the upstroke at low, but not at normal (2 mM), Ca²⁺ in the bath. The magnitude of TTX-sensitive currents recorded with 140 mM Na⁺ was about 20 pA/pF. TTX-sensitive currents decreased five-fold when Ca²⁺ increased from 2 to 10 mM. The currents reduced three-fold in the presence of 10 mM caffeine, but remained unaltered by 1 mM of isobutylmethylxanthine (IBMX). In addition to L-type Ca²⁺ currents (15 pA/pF in 20 mM Ca²⁺), we also found Ca²⁺ currents that are resistant to 10 μM nifedipine (5 pA/pF in 20 mM Ca²⁺). Based on their biophysical properties, these Ca²⁺ currents are likely to be through voltage-gated T-type Ca²⁺ channels. Our results suggest that Na⁺ and at least two types (T- and L-) of Ca²⁺ voltage-gated channels contribute to depolarization of smooth muscle cells in skeletal muscle arterioles. Voltage-gated Na⁺ channels appear to be under a tight control by Ca²⁺ signaling.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Arteriolar oxygen reactivity: where is the sensor and what is the mechanism of action?

Arterioles in the peripheral microcirculation are exquisitely sensitive to changes in PO2 in their environment: increases in PO2 cause vasoconstriction while decreases in PO2 result in vasodilatation. However, the cell type that senses O2 (the O2 sensor) and the signalling pathway that couples changes in PO2 to changes in arteriolar tone (the mechanism of action) remain unclear. Many (but not all) ex vivo studies of isolated cannulated resistance arteries and large, first-order arterioles support the hypothesis that these vessels are intrinsically sensitive to PO2 with the smooth muscle, endothelial cells, or red blood cells serving as the O2 sensor. However, in situ studies testing these hypotheses in downstream arterioles have failed to find evidence of intrinsic O2 sensitivity, and instead have supported the idea that extravascular cells sense O2 . Similarly, ex vivo studies of isolated, cannulated resistance arteries and large first-order arterioles support the hypotheses that O2 -dependent inhibition of production of vasodilator cyclooxygenase products or O2 -dependent destruction of nitric oxide mediates O2 reactivity of these upstream vessels. In contrast, most in vivo studies of downstream arterioles have disproved these hypotheses and instead have provided evidence supporting the idea that O2 -dependent production of vasoconstrictors mediates arteriolar O2 reactivity, with significant regional heterogeneity in the specific vasoconstrictor involved. Oxygen-induced vasoconstriction may serve as a protective mechanism to reduce the oxidative burden to which a tissue is exposed, a process that is superimposed on top of the local mechanisms which regulate tissue blood flow to meet a tissue's metabolic demand.

Hypoxia. 4. Hypoxia and ion channel function

The ability to sense and respond to oxygen deprivation is required for survival; thus, understanding the mechanisms by which changes in oxygen are linked to cell viability and function is of great importance. Ion channels play a critical role in regulating cell function in a wide variety of biological processes, including neuronal transmission, control of ventilation, cardiac contractility, and control of vasomotor tone. Since the 1988 discovery of oxygen-sensitive potassium channels in chemoreceptors, the effect of hypoxia on an assortment of ion channels has been studied in an array of cell types. In this review, we describe the effects of both acute and sustained hypoxia (continuous and intermittent) on mammalian ion channels in several tissues, the mode of action, and their contribution to diverse cellular processes.

Smooth muscle alpha1D-adrenoceptors mediate phenylephrine-induced vasoconstriction and increases in endothelial cell Ca2+ in hamster cremaster arterioles

BACKGROUND AND PURPOSE

alpha(1)-Adrenoceptor agonists induce Ca(2+)-transients in endothelial cells (ECs) of arterioles. However, the presence of alpha(1)-adrenoceptors on arteriolar ECs has not been excluded, and the identity of alpha(1)-adrenoceptor subtypes in arterioles only has been inferred from pharmacology. Therefore, we determined which subtypes were expressed by vascular smooth muscle cells (VSMCs) and ECs, and which subtype mediated alpha(1)-adrenoceptor-induced constriction.

EXPERIMENTAL APPROACH

EC Ca(2+)-transients in isolated, cannulated hamster cremasteric arterioles or freshly isolated ECs were studied using Fura 2. Arteriolar diameter was measured by video microscopy. alpha(1)-Adrenoceptor expression was assessed by western blot of whole-arteriolar homogenates and real-time RT-PCR on enzymatically isolated VSMCs and ECs.

KEY RESULTS

Phenylephrine-induced constriction and EC Ca(2+)-transients were abolished by the alpha(1)-adrenoceptor antagonist prazosin (30 nM) in arterioles. Phenylephrine-induced constriction was inhibited by the alpha(1D)-adrenoceptor antagonist BMY 7378 (K(B)=2.96 nM) and the alpha(1A)-adrenoceptor antagonist 5-methylurapidil (K(B)=4.08 nM), suggesting a significant role for alpha(1D)-adrenoceptors. Western blots confirmed alpha(1D)-adrenoceptor expression, but did not detect alpha(1A)-adrenoceptors. VSMCs expressed alpha(1D)- and alpha(1A)-, but not alpha(1B)-, adrenoceptor transcripts. No alpha(1)-adrenoceptor transcripts were detected in ECs. Neither phenylephrine (10 microM) nor noradrenaline (0.1-1 microM) elicited Ca(2+)-transients in freshly isolated ECs, whereas the endothelium-dependent vasodilators methacholine (1 microM) and substance P (100 nM) consistently increased Ca(2+).

CONCLUSIONS AND IMPLICATIONS

We reject the hypothesis that hamster cremasteric arteriolar ECs express alpha(1)-adrenoceptors and conclude that alpha(1)-adrenoceptor agonists predominantly act on VSMC alpha(1D)-adrenoceptors to cause vasoconstriction and a subsequent rise in EC Ca(2+).

Membrane hyperpolarization is not required for sustained muscarinic agonist-induced increases in intracellular Ca2+ in arteriolar endothelial cells

OBJECTIVE

Hyperpolarization modulates Ca2+ influx during agonist stimulation in many endothelial cells, but the effects of hyperpolarization on Ca2+ influx in freshly isolated arteriolar endothelial cells are unknown. Therefore, the purpose of the present study was to characterize agonist-induced Ca2+ transients in freshly isolated arteriolar endothelial cells and to test the hypothesis that membrane hyperpolarization augments agonist-induced Ca2+ influx into these cells.

METHODS

Arterioles were removed from hamster cremaster muscles and arteriolar endothelial cells were enzymatically isolated. Endothelial cells were loaded with Fura 2-AM and the Fura 2 ratio measured photometrically as an index of intracellular Ca2+. The cells were then stimulated with the muscarinic, cholinergic agonist, methacholine, and the resulting Ca2+ transients were measured.

RESULTS

Methacholine (1 microM) increased the endothelial cell Fura 2 ratio from a baseline of 0.81 +/- 0.02 to an initial peak of 1.17 +/- 0.05 (n = 17) followed by a sustained plateau of 1.12 +/- 0.07. The plateau phase of the Ca2+ transient was inhibited by removal of extracellular Ca2+ (n = 12, p < .05), or the nonselective cation channel blockers Gd3+ (30 microM; n = 7, p < .05) or La3+ (50 microM; n = 7, p < .05) without significant effect on the baseline or peak (p > .05). The initial peak of methacholine-induced Ca2+ transients was inhibited by the IP3-receptor antagonist xestospongin D (10 microM, n = 5, p < .05). The methacholine-induced Ca2+ transients were accompanied by endothelial cell hyperpolarization of approximately 14-18 mV, as assessed by experiments using the potentiometric dye, di-8-ANEPPS as well as by patch-clamp experiments. However, inhibition of hyperpolarization by blockade of Ca2+-activated K+ channels with charybdotoxin (100 nM) and apamin (100 nM) (n = 5), or exposure of endothelial cells to 80 or 145 mM KCl (both n = 7) had no effect on the plateau phase of methacholine-induced Ca2+ transients (p > .05).

CONCLUSIONS

Freshly isolated arteriolar endothelial cells display agonist-induced Ca2+ transients. For the muscarinic agonist, methacholine, these Ca2+ transients result from release of Ca2+ from intracellular stores through IP3 receptors, followed by sustained influx of extracellular Ca2+. While these changes in intracellular Ca2+ are associated with endothelial cell hyperpolarization, the methacholine-induced, sustained increase in intracellular Ca2+ appears to be independent from this change in membrane potential. These data suggest that arteriolar endothelial cells may possess a novel Ca2+ influx pathway, or that the relationship between intracellular Ca2+ and Ca2+ influx is more complex than that observed in other endothelial cells.

K+-induced dilation of hamster cremasteric arterioles involves both the Na+/K+-ATPase and inward-rectifier K+ channels

OBJECTIVE

The mechanism by which elevated extracellular potassium ion concentration ([K+]o) causes dilation of skeletal muscle arterioles was evaluated.

METHODS

Arterioles (n = 111) were hand-dissected from hamster cremaster muscles, cannulated with glass micropipettes and pressurized to 80 cm H2O for in vitro study. The vessels were superfused with physiological salt solution containing 5 mM KCl, which could be rapidly switched to test solutions containing elevated [K+]o and/or inhibitors. The authors measured arteriolar diameter with a computer-based diameter tracking system, vascular smooth muscle cell membrane potential with sharp micropipettes filled with 200 mM KCl, and changes in intracellular Ca2+ concentration ([Ca2+]i) with Fura 2. Membrane currents and potentials also were measured in enzymatically isolated arteriolar muscle cells using patch clamp techniques. The role played by inward rectifier K+ (KIR) channels was tested using Ba2+ as an inhibitor. Ouabain and substitution of extracellular Na+ with Li+ were used to examine the function of the Na+/K+ ATPase.

RESULTS

Elevation of [K+]o from 5 mM up to 20 mM caused transient dilation of isolated arterioles (27 +/- 1 microm peak dilation when [K+]o was elevated from 5 to 20 mM, n = 105, p <.05). This dilation was preceded by transient membrane hyperpolarization (10 +/-1 mV, n = 23, p <.05) and by a fall in [Ca2+]i as indexed by a decrease in the Fura 2 fluorescence ratio of 22 +/- 5% (n = 4, p <.05). Ba(2+) (50 or 100 microM) attenuated the peak dilation (40 +/- 8% inhibition, n = 22) and hyperpolarization (31 +/- 12% inhibition, n = 7, p <.05) and decreased the duration of responses by 37 +/-11% (n = 20, p < 0.05). Both ouabain (1 mM or 100 microM) and replacement of Na+ with Li+ essentially abolished both the hyperpolarization and vasodilation.

CONCLUSIONS

Elevated [K+]o causes transient vasodilation of skeletal muscle arterioles that appears to be an intrinsic property of the arterioles. The results suggest that K+-induced dilation involves activation of both the Na+/K+ ATPase and KIR channels, leading to membrane hyperpolarization, a fall in [Ca2+]i, and culminating in vasodilation. The Na+/K+ ATPase appears to play the major role and is largely responsible for the transient nature of the response to elevated [K+]o, whereas KIR channels primarily affect the duration and kinetics of the response.