To explore possible mechanisms underlying hypoxia-induced pulmonary vasoconstriction, the effect of hypoxia on outward K+ current (Iout) was evaluated in primary cultured rat pulmonary (PA) and mesenteric (MA) arterial smooth muscle cells using the whole cell patch-clamp technique. When the cells were bathed in standard physiological salt solution and the patch pipettes contained Ca(2+)-free media with 10 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), virtually all of the Iout, including both the rapidly inactivating component (Irt) and the steady-state (noninactivating) component (Iss), was mediated by voltage-gated K+ channels. Reduction of O2 tension in the bath solution from 155 Torr to < 74 Torr with sodium dithionite reversibly inhibited both Irt and Iss in PA myocytes, but not in MA myocytes. The hypoxia-sensitive Iss was activated at about -50 mV; thus, some of the channels responsible for this current may be open at the resting membrane potential (-40 +/- 1 mV) of PA cells used in this study. Hypoxia also significantly depolarized PA cells bathed in PSS (1.8 mM Ca2+) from -40.7 +/- 1.3 to -24.0 +/- 2.4 mV, and PA cells bathed in Ca(2+)-free PSS (0.1 mM EGTA) from -38.4 +/- 1.3 to -26.1 +/- 3.9 mV. The hypoxia-induced inhibition of Iout in PA cells was accompanied by an apparent increase in inward Ca2+ current.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of hypoxia on the cytosolic Ca2+ concentration, [Ca2+]i, were characterized in cultured pulmonary arterial smooth muscle (PASM) cells. Reducing O2 tension (PO2) from 150 to < 25 Torr induced a reversible 100-200% increase in [Ca2+]i that was characterized by two components: an early rise in [Ca2+]i that was dependent on the rate, as well as the magnitude, of decline in PO2 and a later, steady-state increase that was independent of the rate at which PO2 changed. Caffeine lowered [Ca2+]i during normoxia and blocked the early component of the response to hypoxia, whereas the steady-state hypoxic response was only partially inhibited. Like hypoxia, thapsigargin (TG) elevated [Ca2+]i, and there was no additional hypoxia-induced elevation in [Ca2+]i at any time after exposure to TG. At steady state, the hypoxic responses were completely reversed by removal of extracellular Ca2+, whereas, on average, verapamil and nifedipine attenuated the hypoxia-induced increases in [Ca2+]i by only 44 and 35%, respectively. These results suggest that hypoxia-induced elevation of [Ca2+]i in PASM cells consists of an early release of Ca2+ from the sarcoplasmic reticulum and a later influx of extracellular Ca2+, in part, through nifedipine- and verapamil-insensitive Ca2+ channels. The results are consistent with the idea that hypoxia and thapsigargin may share common mechanisms for tonically increasing [Ca2+]i.

Regulation of the cytosolic free Ca2+ concentration ([Ca2+]cyt) by an Na/Ca exchanger was studied in primary cultured rat cortical astrocytes. [Ca2+]cyt was measured by digital imaging in cells loaded with fura-2. The resting [Ca2+]cyt, approximately 150 nM, was only slightly increased by reducing the extracellular Na+ concentration ([Na+]o) to 6.2 mM, or by treating the cells with ouabain for 15 min (to raise cytosolic Na+). Following treatment with ouabain, however, lowering [Na+]o caused [Ca2+]cyt to rise rapidly to approximately 1300 nM. When Ca2+ sequestration in intracellular stores was blocked by thapsigargin, lowering [Na+]o increased [Ca2+]cyt to approximately 1500 nM in the absence of ouabain. The low-[Na+]o-stimulated rise in [Ca2+]cyt was abolished by removal of external Ca2+, but was not blocked by the Ca2+ channel blocker verapamil, or by caffeine or ryanodine, which deplete an intracellular Ca2+ store responsible for Ca(2+)-induced Ca2+ release. These data suggest that Na+ gradient reduction promotes net Ca2+ gain via Na/Ca exchange. Normally, however, a large rise in [Ca2+]cyt is prevented by sequestration of the entering Ca2+; this buffering of cytosolic Ca2+ can be circumvented by blocking sequestration with thapsigargin, or overwhelmed by enhancing net Ca2+ gain by pretreating the cells with ouabain. The presence of Na/Ca exchanger protein and mRNA in the astrocytes was confirmed by Western and Northern blot analyses, respectively. Immunohistochemistry revealed that exchanger molecules are distributed in a reticular pattern over the astrocyte surface. We suggest that the Na/Ca exchanger plays a role in regulating both [Ca2+]cyt and the intracellular stores of Ca2+ in astrocytes, and may thus contribute to the control of astrocyte responsiveness to neurotransmitters and neurotoxins.

Regulation of intracellular Na+ ([Na+]i) in cultured vascular smooth muscle cells (A7r5 line) was studied with Na(+)-sensitive fluorescent dye sodium-binding benzofuran isophthalate. Digital imaging microscopy was used to study single-cell fluorescence. Na+ was distributed uniformly in cytoplasm and nucleus; mean Na+ concentration in resting cells was 4.4 +/- 0.3 mM in cytoplasmic areas ([Na+]cyt) and 4.5 +/- 0.4 mM in nuclear areas ([Na+]n). Na+ pump inhibition and cell activation evoked uniform changes in [Na+]cyt and [Na+]n. Inhibition of Na+ pump with 1 mM ouabain or K(+)-free medium caused a rise in [Na+]cyt; in the latter case, [Na+]cyt fell rapidly when external K+ was later restored. Exposure to Ca(2+)-free medium also caused [Na+]cyt to rise; this effect was augmented by Na+ pump inhibition and was reversed by 10(-5) M verapamil or nitrendipine or by restoration of external Ca2+. The implication is that this Na+ entry in absence of external Ca2+ is mediated by Ca2+ channels. Activation by 10(-9) M arginine vasopressin (AVP) and 10(-6) M serotonin (5-HT) caused [Na+]cyt to increase, but response to 5-HT was small (0.6 mM on average) and transient, whereas response to AVP was larger (2.4 mM on average) and was maintained as long as AVP was present (to 20 min). AVP and, to a much smaller extent, 5-HT stimulated Na+ influx; this could be detected when Na+ pump was inhibited by ouabain. Both AVP and 5-HT activated the Na+ pump, as detected by ouabain-sensitive decrease in [Na+]cyt when Na+ influx was inhibited. Agonist-evoked increases in [Na+]cyt were dependent on a rise in cytosolic Ca2+ concentration ([Ca2+]cyt); these [Na+]cyt responses were abolished by prolonged exposure to Ca(2+)-free media, when cytoplasmic Ca2+ was chelated with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, or when Ca2+ mobilization was blocked with thapsigargin. Raising [Ca2+]cyt with 40 mM K+ or with thapsigargin did not increases in [Na+]cyt. We conclude that 1) AVP- and 5-HT-evoked increases in [Na+]cyt are agonist specific and depend on the balance between stimulated Na+ influx and efflux; 2) AVP and 5-HT activate the Na+ pump; this is, at least in part, independent of agonist-induced rise in [Na+]cyt; and 3) a rise in [Ca2+]cyt is necessary but not sufficient to trigger agonist-evoked rise in [Na+]i.

The electrophysiological properties of cultured single vascular smooth muscle (VSM) cells from rat pulmonary (PA) and mesenteric (MA) arteries were studied using the whole cell patch-clamp technique. Cells were studied at 3-7 days as primary cultures, or were replated after 10-20 days and subcultured for 2-5 days. In the standard physiological bath solution (containing 1.8 mM Ca2+), and with 125 mM K+ + 10 mM ethylene glycol-bis(beta-aminoethyl ether)- N,N,N',N'-tetraacetic acid (EGTA)-filled pipettes, both PA and MA primary cultured cells had high input resistances (mean = 2-3 G omega) and resting membrane potentials of about -40 mV. The cells were clamped at a holding potential of -70 mV. Depolarization to -20 mV or more evoked a transient inward current (Iin) that was eliminated in Ca(2+)-free bath solution; this indicates that Iin was carried by Ca2+. Iin was substantially smaller in subcultured cells from both PA and MA. Depolarization also activated three components of outward current (Iout) in primary cultured PA and MA cells: a rapidly inactivating transient component (Irt), a slowly inactivating transient component (Ist), and a steady-state (noninactivating) component (Iss). All three components of Iout were inhibited to varying degrees by 5 mM 4-aminopyridine and were eliminated by replacing intracellular K+ with Cs+, but were only minimally affected by removal of extracellular Ca2+. These results suggest that this Iout was carried by K+ and was voltage gated. Little external Ca(2+)-dependent Iout was observed under these conditions, but a substantial Ca(2+)-dependent component was seen when the EGTA concentration in the pipettes was reduced to 0.1 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of reducing external Na+ ([Na+]o) and increasing internal Na+ ([Na+]i) on evoked tension in arterial rings were correlated with intracellular Ca2+ transients evoked in cultured A7r5 cells. Contractions were elicited in rings of rat aorta and a small mesenteric artery branch by serotonin (5-HT), arginine vasopressin (AVP), or a high K+ concentration ([K+]o). Contractions were augmented by reduction of [Na+]o (replaced by Li+ or N-methylglucamine) or inhibition of the Na+ pump (to raise [Na+]i). These effects were dependent on external Ca2+. Ca2+ transients were measured by digital imaging of fura-2/AM-loaded A7r5 cells. The apparent free Ca2+ concentration [( Ca2+]app) in eight unstimulated cells was 110 +/- 10 nM in the peripheral cytoplasm. Activation of A7r5 cells by 5-HT, AVP, or high [K+]o evoked transient increases in [Ca2+]app. Lowering [Na+]o or raising [Na+]i elevated [Ca2+]app significantly in unstimulated cells and increased the amplitude and duration of the Ca2+ transients. These effects required external Ca2+. Caffeine (5-10 mM) reduced [Ca2+]app in resting cells and attenuated the vasoconstrictor-evoked Ca2+ transients in low [Na+]o. The data imply that Na+ gradient reduction promotes Ca2+ entry and slows Ca2+ extrusion via Na(+)-Ca2+ exchange. This increases cytosolic free Ca2+ and the intracellular Ca2+ stores so that more Ca2+ is delivered to the contractile apparatus during cell activation. Such augmented Ca2+ transients could induce the amplified vasoconstrictor-evoked contractions observed in arterial rings under reduced Na+ gradient conditions.

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