The effect of alkaline pH and transmural pressure on arterial constriction and membrane potential of hypertensive cerebral arteries

pCO(2) and pH regulation of cerebral blood flow

CO₂ serves as one of the fundamental regulators of cerebral blood flow (CBF). It is widely considered that this regulation occurs through pCO₂-driven changes in pH of the cerebral spinal fluid (CSF), with elevated and lowered pH causing direct relaxation and contraction of the smooth muscle, respectively. However, some findings also suggest that pCO₂ acts independently of and/or in conjunction with altered pH. This action may be due to a direct effect of CSF pCO₂ on the smooth muscle as well as on the endothelium, nerves, and astrocytes. Findings may also point to an action of arterial pCO₂ on the endothelium to regulate smooth muscle contractility. Thus, the effects of pH and pCO₂ may be influenced by the absence/presence of different cell types in the various experimental preparations. Results may also be influenced by experimental parameters including myogenic tone as well as solutions containing significantly altered HCO₃⁻ concentrations, i.e., solutions routinely employed to differentiate the effects of pH from pCO₂. In sum, it appears that pCO₂, independently and in conjunction with pH, may regulate CBF.

Loss of cerebrovascular Shaker-type K(+) channels: a shared vasodilator defect of genetic and renal hypertensive rats

The cerebral arteries of hypertensive rats are depolarized and highly myogenic, suggesting a loss of K(+) channels in the vascular smooth muscle cells (VSMCs). The present study evaluated whether the dilator function of the prominent Shaker-type voltage-gated K(+) (K(V)1) channels is attenuated in middle cerebral arteries from two rat models of hypertension. Block of K(V)1 channels by correolide (1 micromol/l) or psora-4 (100 nmol/l) reduced the resting diameter of pressurized (80 mmHg) cerebral arteries from normotensive rats by an average of 28 +/- 3% or 26 +/- 3%, respectively. In contrast, arteries from spontaneously hypertensive rats (SHR) and aortic-banded (Ao-B) rats with chronic hypertension showed enhanced Ca(2+)-dependent tone and failed to significantly constrict to correolide or psora-4, implying a loss of K(V)1 channel-mediated vasodilation. Patch-clamp studies in the VSMCs of SHR confirmed that the peak K(+) current density attributed to K(V)1 channels averaged only 5.47 +/- 1.03 pA/pF, compared with 9.58 +/- 0.82 pA/pF in VSMCs of control Wistar-Kyoto rats. Subsequently, Western blots revealed a 49 +/- 7% to 66 +/- 7% loss of the pore-forming alpha(1.2)- and alpha(1.5)-subunits that compose K(V)1 channels in cerebral arteries of SHR and Ao-B rats compared with control animals. In each case, the deficiency of K(V)1 channels was associated with reduced mRNA levels encoding either or both alpha-subunits. Collectively, these findings demonstrate that a deficit of alpha(1.2)- and alpha(1.5)-subunits results in a reduced contribution of K(V)1 channels to the resting diameters of cerebral arteries from two rat models of hypertension that originate from different etiologies.

Hypertension increases middle cerebral artery resting tone in spontaneously hypertensive rats: role of tonic vasoactive factor availability

The present study explores the contribution of alterations in resting tone to cerebral artery narrowing in SHRs (spontaneously hypertensive rats) and the role of hypertension development. Young pre-hypertensive and adult fully hypertensive SHRs and age-matched Wistar–Kyoto rat controls were used. The contribution of basal vasoactive factors to resting tone was studied in middle cerebral arteries with pressure myography. Basal NO and O2− (superoxide anion) availability were determined with fluorescent indicators using confocal microscopy and lucigenin-enhanced chemiluminescence. Basal O2− was also assessed in mesenteric resistance arteries. Middle cerebral arteries from adult rats, but not young pre-hypertensive rats, had augmented myogenic responses and resting tone and decreased relaxation to sodium nitroprusside compared with their normotensive counterparts. Cerebral arteries from adult SHRs also had an increase in tonic NO associated with a decrease in basal O2− availability. Basal O2− was instead increased in mesenteric arteries from SHRs. The present results indicate that large cerebral arteries from SHRs have an increase in their resting tone as a consequence of sustained hypertension and that this is related to a decrease in NO responsiveness. We suggest that this increase in resting tone and myogenic responses could act as a protective mechanism against the development of stroke in SHRs. The present study also demonstrates some unusual findings regarding the current understanding of the NO/O2− balance in hypertension with important differences between vascular beds and draws attention to the complexity of this balance in cardiovascular health and disease.

Enhanced Stretch-Induced Myogenic Tone in the Basilar Artery of Spontaneously Hypertensive Rats

We investigated if the magnitude of myogenic tone in the basilar artery of SHR differs from that in WKY and, if so, whether RhoA- or PKC-dependent mechanisms were involved. Myogenic tone was developed in response to stretch. Stretch-induced myogenic contraction was significantly greater in the SHR than WKY in the presence of external Ca(2+). However, in the absence of external Ca(2+), stretch did not evoke a myogenic tone. The [Ca(2+)](i)-induced contraction was larger in SHR than WKY and the [Ca(2+)](i)-force curve was significantly shifted to the left in SHR compared to WKY. Y-27632 significantly inhibited stretch-induced myogenic tone, but the inhibitory effect was larger in the SHR than WKY. However, PKC inhibitors had no significant effect on the myogenic tone. RhoA and PKCepsilon were expressed at higher levels in the SHR compared to the WKY. RhoA and PKCalpha translocated from the cytosol to the cell membrane in response to stretch in both animals, but PKCepsilon was translocated only in SHR. Our results strongly suggest that stretch-induced myogenic tone is enhanced in SHR, and the activation of RhoA/Rho kinase plays an important role in the enhanced myogenic tone in SHR.

Relative contribution of Rho kinase and protein kinase C to myogenic tone in rat cerebral arteries in hypertension

Arterial smooth muscle constriction in response to pressure, i.e., myogenic tone, may involve calcium-dependent and calcium-sensitization mechanisms. Calcium sensitization in vascular smooth muscle is regulated by kinases such as PKC and Rho kinase, and activity of these kinases is known to be altered in cardiovascular disorders. In the present study, we evaluated the relative contribution of PKC and Rho kinase to myogenic tone in cerebral arteries in hypertension. Myogenic tone and arterial wall calcium in Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) were measured simultaneously, and the effect of PKC and Rho kinase inhibitors on myogenic tone was evaluated. SHR arteries showed significantly greater myogenic tone than WKY arteries. Pressure/wall tension-arterial wall calcium curves showed a hyperbolic relation in WKY rats, but the curves for SHR arteries were parabolic. Myogenic tone was decreased by the Rho kinase inhibitors Y-27632 and HA-1077, with a significantly greater effect in SHR than in WKY arteries. Reduction in myogenic tone produced by the PKC inhibitor bisindolylmaleimide I in WKY and SHR arteries was significantly less than that produced by Rho kinase inhibition. The pressure-dependent increase in myogenic tone was significantly decreased by Y-27632, and the decrease was markedly greater than that produced by bisindolylmaleimide I in SHR arteries. In WKY arteries, the pressure-dependent increase in myogenic tone was decreased to a similar extent by Y-27632 and bisindolylmaleimide I. These results suggest greater myogenic tone with increased calcium sensitization in SHR arteries, largely because of Rho kinase activation, with a minor contribution of PKC activation.

Cerebral artery responses to pressure and flow in uremic hypertensive and spontaneously hypertensive rats

Impaired cerebral blood flow autoregulation is seen in uremic hypertension, whereas in nonuremic hypertension autoregulation is shifted toward higher perfusion pressure. The cerebral artery constricts in response to a rise in either lumen pressure or flow; we examined these responses in isolated middle cerebral artery segments from uremic Wistar-Kyoto rats (WKYU), normotensive control rats (WKYC), and spontaneously hypertensive rats (SHR). Pressure-induced (myogenic) constriction developed at 100 mmHg; lumen flow was then increased in steps from 0 to 98 microl/min. Some vessels were studied after endothelium ablation. Myogenic constriction was significantly lower in WKYU (28 +/- 2.9%) compared with both WKYC (39 +/- 2.5%, P = 0.035) and SHR (40 +/- 3.1%, P = 0.018). Flow caused constriction of arteries from all groups in an endothelium-independent manner. The response to flow was similar in WKYU and WKYC, whereas SHR displayed increased constriction compared with WKYU (P < 0.001) and WKYC (P < 0.001). We conclude that cerebral myogenic constriction is decreased in WKYU, whereas flow-induced constriction is enhanced in SHR.

Alkaline pH shifts Ca2+ sparks to Ca2+ waves in smooth muscle cells of pressurized cerebral arteries

The effects of external pH (7.0-8.0) on intracellular Ca(2+) signals (Ca(2+) sparks and Ca(2+) waves) were examined in smooth muscle cells from intact pressurized arteries from rats. Elevating the external pH from 7.4 to 7.5 increased the frequency of local, Ca(2+) transients, or "Ca(2+) sparks," and, at pH 7.6, significantly increased the frequency of Ca(2+) waves. Alkaline pH-induced Ca(2+) waves were inhibited by blocking Ca(2+) release from ryanodine receptors but were not prevented by inhibitors of voltage-dependent Ca(2+) channels, phospholipase C, or inositol 1,4,5-trisphosphate receptors. Activating ryanodine receptors with caffeine (5 mM) at pH 7.4 also induced repetitive Ca(2+) waves. Alkalization from pH 7.4 to pH 7.8-8.0 induced a rapid and large vasoconstriction. Approximately 82% of the alkaline pH-induced vasoconstriction was reversed by inhibitors of voltage-dependent Ca(2+) channels. The remaining constriction was reversed by inhibition of ryanodine receptors. These findings indicate that alkaline pH-induced Ca(2+) waves originate from ryanodine receptors and make a minor, direct contribution to alkaline pH-induced vasoconstriction.

Invited review: arteriolar smooth muscle mechanotransduction: Ca(2+) signaling pathways underlying myogenic reactivity

The smooth muscle of arterioles responds to an increase in intraluminal pressure with vasoconstriction and with vasodilation when pressure is decreased. Such myogenic vasoconstriction provides a level of basal tone that enables arterioles to appropriately adjust diameter in response to neurohumoral stimuli. Key in this process of mechanotransduction is the role of changes in intracellular Ca(2+). However, it is becoming clear that considerable complexity exists in the spatiotemporal characteristics of the Ca(2+) signal and that changes in intracellular Ca(2+) may play roles other than direct effects on the contractile process via activation of myosin light-chain phosphorylation. The involvement of Ca(2+) may extend to modulation of ion channels and release of Ca(2+) from the sarcoplasmic reticulum, alterations in Ca(2+) sensitivity, and coupling between cells within the vessel wall. The purpose of this brief review is to summarize the current literature relating to Ca(2+) and the arteriolar myogenic response. Consideration is given to coupling of Ca(2+) changes to the mechanical stimuli, sources of Ca(2+), involvement of ion channels, and spatiotemporal aspects of intracellular Ca(2+) signaling.

Extracellular pH, Ca(2+) influx, and response of vascular smooth muscle cells to 5-hydroxytryptamine

BACKGROUND AND PURPOSE

Cerebral vascular smooth muscle cells (VSMCs) contract on extracellular pH (pH(o)) increases and relax on pH(o) decreases. These changes in tone are believed to result from changes in [Ca(2+)](i), although the responsible mechanisms are not fully understood. VSMCs also contract in response to 5-hydroxytryptamine (5-HT), which increases [Ca(2+)](i) via both Ca(2+) release and influx. We hypothesized that examining effects of pH(o) decreases on 5-HT-induced [Ca(2+)](i) changes would allow us to identify mechanisms whereby pH(o) influences tone. Accordingly, we compared [Ca(2+)](i) increases in cerebral VSMCs, evoked by 5-HT, with increases evoked by increased pH(o) and examined 5-HT-dependent [Ca(2+)](i) increases at normal and decreased pH(o).

METHODS

We monitored [Ca(2+)](i,), using the Ca(2+)-sensitive dye fura 2, in cultured rat cerebral VSMCs obtained by enzymatic digestion of middle cerebral arteries and their branches (passages 1 to 3) grown on glass coverslips and superfused with physiological saline.

RESULTS

Increasing pH(o) from 7.3 to 7.8 increased [Ca(2+)](i), and these increases were prevented in Ca(2+)-free solutions. Decreasing pH(o) from 7.3 to 6.9 did not alter [Ca(2+)](i) unless [Ca(2+)](i) was first raised by treatment with 5-HT (10 micromol/L). 5-HT resulted in biphasic [Ca(2+)](i) increases characterized by transient peaks blocked by the Ca(2+)-ATPase inhibitor thapsigargin (10 nmol/L) and prolonged plateaus blocked by the Ca(2+) channel blocker Ni(2+) (1 mmol/L). Acidification did not alter the transient peaks but significantly reduced 5-HT-induced Ca(2+) influx.

CONCLUSIONS

We conclude that increasing pH(o) induces Ca(2+) influx in rat cerebral VSMCs and decreasing pH(o) inhibits 5-HT-stimulated Ca(2+) entry but not intracellular Ca(2+) release.

Hypocapnic constriction in rabbit basilar artery in vitro: triggering by N(G)-monomethyl-L-arginine monoacetate and dependence on endothelin-1 and alkalosis

This study tested whether hypocapnic constriction of the rabbit basilar artery in vitro can be triggered by a nitric oxide (NO) synthase inhibitor, and whether the resulting constriction is (1) due to the alkaline pH associated with hypocapnia, and (2) endothelin-1 mediated. Hypocapnic (25 mM NaHCO(3); pH 7.76; pCO(2) 14.2) or isocapnic alkaline solution (50 mM NaHCO(3); pH 7.73; pCO(2) 35.0) rarely altered basal tension. N(G)-monomethyl-L-arginine monoacetate (L-NMMA; 0.1 mM) challenge in hypocapnic or isocapnic alkaline solution resulted in near maximal tension that was maintained for 2-2.5 h even following L-NMMA washout. L-NMMA challenge in normal solution (25 mM NaHCO(3); pH 7. 42; pCO(2) 36.9) also induced near maximal tension, although the tension was maintained for only 25 min (mean). Ac-D-Bhg-L-Leu-Asp-L-Ile-L-Ile-L-Trp (PD145065), homopiperidinyl-CO-Leu-D-Trp(CHO)-D-Trp (BQ610), and N-cis-2, 6-dimethyl-piperidinocarbonyl L-gamma-MeLeu-D-Trp (COOCH(3))-Nle (BQ788; 1-3 microM), endothelin ET(A)/ET(B), endothelin ET(A), and endothelin ET(B) receptor antagonists, respectively, completely relaxed the tension that resulted from L-NMMA challenge in hypocapnic or isocapnic alkaline solution. These results demonstrate that constriction due to hypocapnia in vitro can be triggered by an NO synthase inhibitor and is endothelin-1 mediated. Additionally, alkaline pH in the absence of decreased pCO(2) is sufficient to elicit the constriction.

Influence of pH changes on the actions of verapamil on vascular excitation-contraction coupling

We have previously shown that pH changes alter the cardiovascular responses to verapamil in rat, in vivo and in isolated rat heart. The current study investigated the influence of pH changes on the actions of verapamil on potassium- or noradrenaline-stimulated contraction in rat tail arteries. The proximal 2-2.5 cm of ventral tail artery was bathed in and perfused initially (20-25 min) with physiological salt solution (pH 7.4) which was later made calcium-free at pH 7.4 (control), pH 7.2 (acidosis) or pH 7.67 (alkalosis). After equilibration each artery was exposed to verapamil following which the contractile responses to increasing concentrations of calcium were recorded. The patterns of responses in noradrenaline- or potassium-stimulated arteries were different. In normal conditions, the vasodilator effect of verapamil was predominant in potassium-stimulated arteries but less in the noradrenaline-stimulated preparations. With pH changes the effect of verapamil was enhanced more in noradrenaline- than in potassium-stimulated arteries. It is postulated that pathology-induced changes in the character of calcium channels could alter the effect of calcium channel blockers.

Effects of extravascular acidification and extravascular alkalinization on constriction and depolarization in rat cerebral arterioles in vitro

The relationship between cell membrane potential, vessel diameter, and pH in small cerebral arterioles is not completely understood. This study involved direct, simultaneous measurement of cell membrane potential and vessel diameter at various extracellular pH levels. Arterioles ranging from 44 to 91 microns in diameter were isolated, transferred to a temperature-controlled microscope chamber, which was used as an organ bath, and observed through an inverted videomicroscope. Two vessel cannulation procedures were used: a single-sided cannulation with the other side occluded, and a double-sided and perfused cannulation. After cannulation, the vessels were pressurized to 60 mm Hg intraluminally and the bath temperature was raised to 37 degrees C. Cell membrane potentials of vessel wall cells were obtained after the bath temperature reached 37 degrees C with the vessels partly constricted and again after spontaneous tone (constriction) of the healthy vessels had developed. The effect of extraluminal pH on cell membrane potentials was studied by changing the bath pH from 7.3 to either 7.65 or 6.8 in the single-sided cannulation. The average cell membrane potential for vessels at 37 degrees C, with 60 mm Hg of intraluminal pressure and pH 7.3, was -37.5 mV. The cell membrane potential depolarized to -30.9 mV at pH 7.65 and hyperpolarized to -58.4 mV at pH 6.8, with a slope of 25.8 mV/pH unit. The effect of depolarizing extracellular potassium ions on the cell membrane potential was examined by perfusing two vessels with modified Ringer's solution containing 70 mM KCl. This perfusion method decreased the vessel diameter by 48% and depolarized the observed cell membrane potential from -41.9 to -19.8 mV, with a slope of -0.42 mV per percentage diameter change. These data provide the first measurements of membrane potentials of isolated penetrating arteriole wall cells in vitro. The results indicate that the cell membrane potential relates linearly to the vessel diameter. This new technique opens the possibility for studying vessel response to stimuli under controlled conditions and regulatory mechanisms such as the propagation of vasomotor responses.

Calcium channel current of vascular smooth muscle cells: extracellular protons modulate gating and single channel conductance

Modulation of L-type Ca2+ channel current by extracellular pH (pHo) was studied in vascular smooth muscle cells from bovine pial and porcine coronary arteries. Relative to pH 7.4, alkaline pH reversibly increased and acidic pH reduced ICa. The efficacy of pHo in modulating ICa was reduced when the concentration of the charge carrier was elevated ([Ca2+]o or [Ba2+]o varied between 2 and 110 mM). Analysis of whole cell and single Ca2+ channel currents suggested that more acidic pHo values shift the voltage-dependent gating (approximately 15 mV per pH-unit) and reduce the single Ca2+ channel conductance gCa due to screening of negative surface charges. pHo effects on gCa depended on the pipette [Ba2+] ([Ba2+]p), pK*, the pH providing 50% of saturating conductance, increased with [Ba2+]p according to pK* = 2.7-2.log ([Ba2+]p) suggesting that protons and Ba2+ ions complete for a binding site that modulates gCa. The above mechanisms are discussed in respect to their importance for Ca2+ influx and vasotonus.

Force, membrane potential, and [Ca2+]i during activation of rat mesenteric small arteries with norepinephrine, potassium, aluminum fluoride, and phorbol ester. Effects of changes in pHi

In activated rat mesenteric small arteries, the effect of pHi on force, membrane potential, and free cytosolic calcium ([Ca2+]i) was assessed. Arteries were mounted in a myograph for isometric force development, and [Ca2+]i, pHi, or membrane potential was measured simultaneously with force. During activation with norepinephrine, potassium, aluminum fluoride (AlF-4), and phorbol 12-myristate 13-acetate (PMA, a phorbol ester), the vessels depolarized and [Ca2+]i increased, although the ratio of force to [Ca2+]i was less during potassium activation than with the other types of activation. Changes in pHi, with a constant pHo, were induced with NH4Cl or by changing PCO2. In resting vessels, the effects of the changes in pHi on tension, membrane potential, and [Ca2+]i were negligible. In vessels activated with norepinephrine or AlF-4, alkalinization caused an acute decrease of tone, which could be explained by a decrease in [Ca2+]i consequent to repolarization of the membrane. In vessels activated with potassium or PMA, the effects of alkalinization were smaller. This is consistent with acute alkalinization, affecting steps proximal in the excitation-contraction coupling distal to activation of G proteins. Acidification caused a transient increase in tone and [Ca2+]i, irrespective of the mode of stimulation, without affecting the membrane potential. Ryanodine did not abolish the transient increase in tone and [Ca2+]i. Thus, acute intracellular acidification may induce tone by release of an intracellular ryanodine-insensitive calcium pool or by affecting transmembranal calcium flux although in a membrane potential-independent way.

Evidence for Na/H exchange and Cl/HCO3 exchange in A10 vascular smooth muscle cells

In the present study we used the pH sensitive absorbance of 5(and6)-carboxy-4',5'-dimethylfluorescein to investigate intracellular pH (pHi) regulation in A10 vascular smooth muscle cells: (1) The steady state pHi in A10 cells averaged 7.01 +/- 0.1 (mean +/- SEM, n = 26) at an extracellular pH of 7.4 (28 mM HCO3/5% CO2). (2) Removal of extracellular sodium led to an intracellular acidification of 0.36 +/- 0.07 pH-units (mean +/- SEM, n = 8). (3) pHi-Recovery after an acute intracellular acid load (by means of NH4Cl-prepulse) was reversibly blocked by 1 mM amiloride and was dependent on the presence of sodium. The velocity of pHi recovery increased with increasing sodium concentrations with an apparent Km for external sodium of about 30 mM and a Vmax of about 0.35 pH units/min. These findings are compatible with a Na/H exchanger being responsible for pHi recovery after an acid load. (4) Removal of extracellular chloride induced an intracellular alkalinization of 0.23 +/- 0.03 pH-units (mean +/- SEM, n = 10). The alkalinization was dependent on the presence of extracellular bicarbonate. (5) Removal of chloride during pHi recovery from an alkaline load (imposed by acetate prepulse) stopped and reversed pHi backregulation. Chloride removal had no effect in the absence of bicarbonate or in the presence of 10(-4) M DIDS, suggesting that the effects were mediated by a Cl/HCO3 exchanger. In conclusion we have demonstrated evidence for a Na/H exchanger and a Cl/HCO3 exchanger in A10 vascular smooth muscle cells.