Effects of pH on vascular tone in rabbit basilar arteries

Abstract

Effects of pH on vascular tone and L-type Ca2+ channels were investigated using Mulvany myograph and voltage-clamp technique in rabbit basilar arteries. In rabbit basilar arteries, high K+ produced tonic contractions by 11+/-0.6 mN (mean+/-S.E.,n=19). When extracellular pH (pHo) was changed from control 7.4 to 7.9 ([alkalosis]o), K+-induced contraction was increased to 128+/-2.1% of the control (n=13). However, K+-induced contraction was decreased to 73+/-1.3% of the control at pHo 6.8 ([acidosis] o, n=4). Histamine (10 microM) also produced tonic contraction by 11+/-0.6 mN (n=17), which was blocked by post-application of nicardipine (1 microM). [alkalosis]o and [acidosis]o increased or decreased histamine-induced contraction to 134+/-5.7% and 27+/-7.6% of the control (n=4, 6). Since high K+- and histamine-induced tonic contractions were affected by nicardipine and pHo, the effect of pHo on voltage-dependent L-type Ca2+ channel (VDCCL) was studied. VDCCL was modulated by pHo: the peak value of Ca2+ channel current (IBa) at a holding of 0 mV decreased in [acidosis]o by 41+/-8.8%, whereas that increased in [alkalosis]o by 35+/-2.1% (n=3). These results suggested that the external pH regulates vascular tone partly via the modulation of VDCC in rabbit basilar arteries.

Fig. 1

Effects of the alteration of the extracellular pH (pH_o) on high K⁺-induced contraction in basilar arteries of rabbits. Mulvany myograph was used for recording isometric tension of rabbit basilar artery. All contractile experiments in this study were done in the presence of nitro-L-arginine (L-NNA, 100 µM). (A) Superfusion of 50 mM of high K⁺ solution to the bath provoked tonic contraction. This contraction was enhanced by pH_o 7.9 in a reversible manner. (B, C) Under acidic condition such as pH_o 7.0 or 6.8 tonic contraction was suppressed in a reversible manner. (D) Bar graphs show mean relative K⁺-induced contraction by alteration of pH_o. Asterisks indicate the data which were considered to be significantly different from control data (^**p<0.01).

Fig. 2

Effects of the repetitive alteration of pH_o on high K⁺-induced contraction in rabbit basilar artery. Modulatory effects on high K⁺-induced contraction by the alteration of pH_o was monitored by repetitive changes of pH_o in rabbit basilar artery. (A) 50 mM of high K⁺-induced tonic contraction was repeatedly increased by pH_o 7.9. (B) Acidotic and alkalotic conditions were induced to 50 mM of high K⁺-induced tonic contraction. Each condition of pH_o 6.8 and 7.9 in a same tissue decreased and increased tonic contraction, respectively, in a reversible manner. (C) In the presence of L-NNA, enhancing effects of alkalotic condition on high K⁺-induced tonic contraction was studied by pre-application of pH_o 7.9. Tonic contraction induced by application of 50 mM of high K⁺ solution (pH_o 7.9) was decreased by post application of normal pH (pH 7.4).

Fig. 3

Histamine-induced contraction in rabbit basilar artery. (A) Histamine (1-20 µM) produced contractions in a concentration-dependent manner. Approximately, 10 µM of histamine produced maximal contraction. (B) In some cases, application of histamine evoked regular small oscillatory phasic contractions. (C) Histamine-induced tonic contraction was suppressed by post-application of nicardipine (1 µM).

Fig. 4

Effects of alteration of pH_o on the histamine-induced contraction in basilar arteries of rabbits. 10 µM of histamine produced tonic contractions in rabbit basilar artery. (A, B) Histamine-induced contraction was decreased or increased by pH_o 6.8 or pH_o 7.9 in a reversible manner. (C) Bar graphs show mean relative histamine-induced contraction by alteration of pH_o. Asterisks indicate the data which were considered to be significantly different from control data (^**p<0.01).

Fig. 5

Effects of pH_o in the presence of nicardipine on High K⁺- and histamine-induced Contraction in Rabbit Basilar Arteries. In A, B, effects of nicardipine and [alkalosis]_o in the presence of nicardipine on high K⁺- and histamine-induced contractions were summarized. Pannel A shows nicardipine completely suppressed high K⁺-induced sustained contraction and these effect was not reversed by [alkalosis]_o significantly. In pannel B, nicardipine suppressed histamine-induced sustained contraction and it was not reversed by [alkalosis]_o significantly. Asterisks indicate the data which were considered to be significantly different from control data (^**p<0.01).

Fig. 6

Effects of alteration of pH_o on Ca²⁺ channel current (I_Ba) in rabbit basilar artery. The membrane potential was held at -80 mV and 2 mM Ca²⁺ was used as a charge carrier. Step 10 mV depolarizing pulses from -50 to +60 mV were applied for 500 msec. (A) Depolarizing pulses above -30 mV elicited inward currents (I_Ca) and nicardipine (1 µM) almost completely blocked I_Ca. (B) Current/voltage (I/V) relationships of I_Ca were obtained in the absence and presence of nicardipine (1 µM). In I/V relationships, I_Ca was significantly suppressed by nicardipine throughout the whole test potential range. 10 mM Ba²⁺ was used as a charge carrier for the study of regulation of Ca²⁺ channel current (I_Ba) by pH_o. In C, D effects of alteration of pH_o on I_Ba were studied. Step depolarizing pulse from -80 to 0 mV were applied for 500 msec every 15 sec. When normal external solution (pH_o 7.4) was changed to pH_o 7.9, I_Ba was increased in a reversible manner (raw traces of I_Ba, see insets). However, pH_o 6.8 decreased I_Ba in a reversible manner (raw traces of I_Ba, insets).