NH4Cl-induced contraction of porcine coronary artery involves activation of dihydropyridine-sensitive Ca2+ entry

Cephalic and saphenous venous blood collected by continuous heating of the paws compared with arterial blood for measurement of blood gas values in well-perfused dogs

BACKGROUND

"Arterialization" of the dorsal hand vein is well-established in human medicine, but not in veterinary medicine.

OBJECTIVES

To compare cephalic and saphenous venous blood collected by continuously heating the paws to 37°C ("arterialization"), with arterial blood (AB) for measurement of blood gas variables in well-perfused dogs.

ANIMALS

Eight healthy dogs.

METHODS

Experimental study. Fore and hind paws were continuously heated to 37°C to "arterialize" cephalic and saphenous venous blood. AB and "arterialized" cephalic and saphenous venous blood (ACV and ASV, respectively) were simultaneously collected from lightly anesthetized dogs with induced metabolic and respiratory acid-base disorders. The pH, partial pressures of carbon dioxide (PCO2 ) and oxygen (PO2 ), bicarbonate concentration [HCO3 - ], and base excess (BE) were measured once in each state. Systolic blood pressure was maintained above 100 mm Hg. The AB, ACV, and ASV values were compared.

RESULTS

The pH, [HCO3 - ], and BE values had no significant difference and good agreement, the PCO2 values had a strong correlation (correlation coefficient of .91-1.00), and the PO2 values had a significant difference (P < .01) and poor agreement between AB and ACV, and between AB and ASV. The PCO2 values of ASV overestimated those of AB by ~3.0 mm Hg, which was considered within clinically allowable limits, while those of ACV were not within clinically allowable limits.

CONCLUSIONS AND CLINICAL IMPORTANCE

Under experimental conditions, the ASV samples were more identical to the AB samples than the ACV samples for pH, PCO2 , [HCO3 - ], and BE values in well-perfused dogs. The saphenous vein is suitable for "arterialization."

Na(+)/H(+) exchange and hypoxic pulmonary hypertension

Intracellular pH (pHi) homeostasis is key to the functioning of vascular smooth muscle cells, including pulmonary artery smooth muscle cells (PASMCs). Sodium-hydrogen exchange (NHE) is an important contributor to pHi control in PASMCs. In this review, we examine the role of NHE in PASMC function, in both physiologic and pathologic conditions. In particular, we focus on the contribution of NHE to the PASMC response to hypoxia, considering both acute hypoxic pulmonary vasoconstriction and the development of pulmonary vascular remodeling and pulmonary hypertension in response to chronic hypoxia. Hypoxic pulmonary hypertension remains a disease with limited therapeutic options. Thus, this review explores past efforts at disrupting NHE signaling and discusses the therapeutic potential that such efforts may have in the field of pulmonary hypertension.

Intracellular pH as a determinant of vascular smooth muscle function

Intracellular pH (pHi) is a physiological parameter that is intimately linked to contractility, growth and proliferation of vascular smooth muscle (VSM). Regarding contractility, no general unifying concept of pHi regulation but a rather complex relation between pHi signals and vascular tone has been revealed so far. The modulation of vasotone by pHi depends on the type of blood vessel as well as on the pattern of regulatory input signals. In addition, changes in pHi have been recognized as an important cellular signal to determine the fate of cells in terms of proliferation or apoptosis. Cellular sensors for pHi include a variety of ion transport systems which control intracellular Ca2+ gradients and are likely to serve as a link between pHi and cell functions. Here we provide an overview on the potential targets and mechanisms that transduce pHi signals in VSM. The role of pHi-sensing signaling complexes and localized pHi signaling as the basis of diversity of pHi regulation of VSM function is discussed.

Intracellular alkalinization augments capacitative Ca2+ entry in vascular smooth muscle cells

Agonist-induced Ca2+ influx of vascular smooth muscle cells is thought to be triggered by depletion of intracellular Ca2+ stores. This study investigated the effects of intracellular alkalinization on capacitative Ca2+ entry in A7r5 rat aortic smooth muscle cells. Intracellular alkalinization was induced by NH(4)Cl. Transplasmalemmal Ca2+ influx due to Ca2+ store depletion induced by thapsigargin, which was abolished by pretreatment of the cells with SKF-96365 but not affected by that with verapamil, was significantly increased by pretreatment with NH(4)Cl. Neither 5-hydroxytryptamine-induced inositol monophosphate accumulation nor intracellular Ca2+ release from its stores was affected by NH(4)Cl. These results suggest that intracellular alkalinization acts on the process(es) after depletion of Ca2+ stores and facilitates capacitative Ca2+ entry in vascular smooth muscle cells.

Crosstalk between voltage-independent Ca2+ channels and L-type Ca2+ channels in A7r5 vascular smooth muscle cells at elevated intracellular pH: evidence for functional coupling between L-type Ca2+ channels and a 2-APB-sensitive cation channel

This study was designed to investigate the role of voltage-independent and voltage-dependent Ca2+ channels in the Ca2+ signaling associated with intracellular alkalinization in A7r5 vascular smooth muscle cells. Extracellular administration of ammonium chloride (20 mmol/L) resulted in elevation of intracellular pH and activation of a sustained Ca2+ entry that was inhibited by 2-amino-ethoxydiphenyl borate (2-APB, 200 micromol/L) but not by verapamil (10 micro;mol/L). Alkalosis-induced Ca2+ entry was mediated by a voltage-independent cation conductance that allowed permeation of Ca2+ (PCa/PNa approximately 6), and was associated with inhibition of L-type Ca2+ currents. Alkalosis-induced inhibition of L-type Ca2+ currents was dependent on the presence of extracellular Ca2+ and was prevented by expression of a dominant-negative mutant of calmodulin. In the absence of extracellular Ca2+, with Ba2+ or Na+ as charge carrier, intracellular alkalosis failed to inhibit but potentiated L-type Ca2+ channel currents. Inhibition of Ca2+ currents through voltage-independent cation channels by 2-APB prevented alkalosis-induced inhibition of L-type Ca2+ currents. Similarly, 2-APB prevented vasopressin-induced activation of nonselective cation channels and inhibition of L-type Ca2+ currents. We suggest the existence of a pH-controlled Ca2+ entry pathway that governs the activity of smooth muscle L-type Ca2+ channels due to control of Ca2+/calmodulin-dependent negative feedback regulation. This Ca2+ entry pathway exhibits striking similarity with the pathway activated by stimulation of phospholipase-C-coupled receptors, and may involve a similar type of cation channel. We demonstrate for the first time the tight functional coupling between these voltage-independent Ca2+ channels and classical voltage-gated L-type Ca2+ channels.

Intracellular alkalinization augments alpha(1)-adrenoceptor-mediated vasoconstriction by promotion of Ca(2+) entry through the non-L-type Ca(2+) channels

Modulation by intracellular pH of the vasoconstriction induced by alpha-adrenoceptor agonists was investigated in isolated guinea pig aorta. NH(4)Cl (15 mM) increased intracellular pH of aortic smooth muscle cells by about 0.2 pH unit and significantly augmented KCl-induced contraction of aortic strips, whereas simultaneous administration of NH(4)Cl (15 mM) plus Na(+) propionate (30 mM) failed to affect intracellular pH or contractility. NH(4)Cl (15 mM) potentiated contractions induced by alpha-adrenoceptor agonists, norepinephrine, phenylephrine and clonidine. Contraction induced by alpha(1)-selective adrenoceptor agonist, phenylephrine, but not that induced by norepinephrine or clonidine, was insensitive to inhibition by verapamil (1 microM). Phenylephrine-induced contraction was not affected by NH(4)Cl in Ca(2+)-free medium whereas extracellular Ca(2+)-induced contraction of phenylephrine-stimulated aorta was significantly augmented by NH(4)Cl. Consistently, 45Ca(2+)uptake into phenylephrine 1 microM)-stimulated aortic strips was increased by incubation with NH(4)Cl. The potentiating effects of NH(4)Cl on both phenylephrine-induced Ca(2+) entry and contraction were antagonized by Na(+) propionate. These results suggest that intracellular alkalinization facilitates alpha(1)-adrenoceptor-mediated vasoconstriction by facilitation of an agonist-induced Ca(2+) entry pathway that is independent of L-type Ca(2+) channels.

Essential role of the beta subunit in modulation of C-class L-type Ca2+ channels by intracellular pH

Elevation of intracellular pH (pHi) enhances the activity of native L-type Ca2+ channels in cardiac and smooth muscle. We studied the modulation by pHi of expressed L-type Ca2+ channels comprised of either the alpha1c subunits alone or of alpha1c plus beta2a subunits. Ca2+ channels were expressed in human embryonic kidney cells (HEK 293) and pHi was increased from a basal level of 7.3 to 8.3 by exposure of cells to NH4Cl (20 mM) or by elevation of extracellular pH to 8.5. Elevation of pHi enhanced the activity of Ca2+ channels derived by coexpression of alpah1c and beta2a subunits. This alkalosis-induced stimulation of channel activity was mainly due to an increase in channel availability. Channels derived by expression of alpha1c alone were not affected by intracellular alkalosis. Our results demonstrate that the pHi sensitivity of L-type Ca2+ channels is conferred by the beta subunit of the channel complex.

Divergent effects of extracellular and intracellular alkalosis on Ca2+ entry pathways in vascular endothelial cells

Modulation by alkalosis of basal leak Ca2+ entry and store-depletion-induced Ca2+ entry was investigated in the vascular endothelial cell line ECV 304. Ca2+ entry was monitored as the increase in the intracellular free Ca2+ concentration ([Ca2+]i) induced by elevation of the extracellular Ca2+ concentration. When ECV 304 cells were challenged with 100 nM thapsigargin in nominally Ca2+-free solution, [Ca2+]i increased transiently, and the increase in [Ca2+]i during a subsequent cumulative elevation of extracellular Ca2+ (from nominally Ca2+-free up to 5 mM) was markedly enhanced compared with non-stimulated cells (i.e. basal Ca2+ leak). Prolonged elevation of the extracellular pH (pHo) from 7.4 to 7.9 did not affect resting [Ca2+]i or the thapsigargin-induced [Ca2+]i transient evoked in nominally Ca2+-free solution, but increased leak Ca2+ entry as well as store-depletion-activated Ca2+ entry significantly. Basal Ca2+ leak and store-depletion-activated Ca2+ entry were enhanced either by acute elevation of pHo from 7.4 to 7.9 or by chronic alkalosis (pHo=7.9). Stimulation of Ca2+ entry by extracellular alkalosis was observed both in normal and in high extracellular K+ (110 mM) solution, suggesting that the effects of alkalosis are independent of membrane potential. The intracellular pH (pHi) increased slightly during both acute and chronic extracellular alkalosis (from 7.22+/-0.01 to 7.37+/-0.04 and 7. 45+/-0.05 respectively). Elevation of pHi to 7.60+/-0.06 at constant pHo by administration of 20 mM NH4Cl failed to stimulate, and in fact inhibited, store-depletion-activated Ca2+ entry. Our results demonstrate that a decrease in the extracellular but not the intracellular proton concentration promotes both basal and stimulated Ca2+ entry into endothelial cells.