Modulation of volume-sensitive chloride current by noradrenaline in rabbit portal vein myocytes.

The effect of noradrenaline on the volume-sensitive chloride current (I(Cl(swell))) was studied with conventional whole-cell recording techniques in freshly dispersed isolated smooth muscle cells of the rabbit portal vein. In the absence of receptor antagonists, noradrenaline produced an increase in the amplitude of I(Cl(swell)) in some cells and a decrease in others. In the presence of the beta-adrenoceptor antagonist propranolol, noradrenaline increased I(Cl(swell)) and in the presence of the alpha(1)-adrenoceptor antagonist prazosin, noradrenaline reduced I(Cl(swell).) The phospholipase C (PLC) inhibitor U73122 reduced the amplitude of I(Cl(swell)) whereas the inactive analogue U73343 had no effect. The phorbol esters phorbol-12-myristate-13-acetate (PMA) and phorbol-12,13-dibutyrate (PDBu) increased the amplitude of I(Cl(swell)) by approximately 60 and 100 %, respectively, in a voltage-independent fashion. Inhibitors of protein kinase C (PKC) chelerythrine and calphostin-C decreased the amplitude of I(Cl(swell)) in a concentration-dependent but voltage-independent manner. Bath application of 8-Br-cAMP decreased I(Cl(swell)) by about 60 % whereas the inhibitor of protein kinase A (PKA) KT5720 increased the amplitude of I(Cl(swell)) by approximately 80-90 %. In the presence of propranolol, chelerythrine prevented the increase of I(Cl(swell)) by noradrenaline; in the presence of prazosin, KT5720 blocked the inhibitory action of noradrenaline. The results show that in rabbit portal vein myocytes noradrenaline enhances I(Cl(swell)) by acting on alpha(1)-adrenoceptors and reduces I(Cl(swell)) by stimulating beta-adrenoceptors. The data suggest that the potentiating and inhibitory effects of noradrenaline are mediated, respectively, by PKC and PKA.

Mechanisms of hydralazine induced vasodilation in rabbit aorta and pulmonary artery.

1. The directly acting vasodilator hydralazine has been proposed to act at an intracellular site in vascular smooth muscle to inhibit Ca(2+) release. 2. This study investigated the mechanism of action of hydralazine on rabbit aorta and pulmonary artery by comparing its effects on the tension generated by intact and beta-escin permeabilized vessels and on the cytoplasmic Ca(2+) concentration, membrane potential and K(+) currents of isolated vascular smooth muscle cells. 3. Hydralazine relaxed pulmonary artery and aorta with similar potency. It was equally effective at inhibiting phasic and tonic contractions evoked by phenylephrine in intact vessels and contractions evoked by inositol 1,4,5 trisphosphate (IP(3)) in permeabilized vessels. 4. Hydralazine inhibited the contraction of permeabilized vessels and the increase in smooth muscle cell Ca(2+) concentration evoked by caffeine with similar concentration dependence, but with lower potency than its effect on IP(3) contractions. 5. Hydralazine had no effect on the relationship between Ca(2+) concentration and force generation in permeabilized vessels, but it slowed the rate at which maximal force was developed before, but not after, destroying sarcoplasmic reticulum function with the calcium ionophore, ionomycin. 6. Hydralazine had no effect on membrane potential or the amplitudes of K(+) currents recorded from isolated smooth muscle cells over the concentration range causing relaxation of intact vessels. 7. The results suggest that the main action of hydralazine is to inhibit the IP(3)-induced release of Ca(2+) from the sarcoplasmic reticulum in vascular smooth muscle cells.

Dual modulation of swelling-activated chloride current by NO and NO donors in rabbit portal vein myocytes.

1. The effects of authentic NO and the NO donor S-nitroso-N-acetylpenicillamine (SNAP) on swelling-activated chloride currents (Iswell) were investigated in freshly dispersed rabbit portal vein smooth muscle cells. Iswell was recorded with the perforated patch configuration of the whole-cell patch clamp technique. 2. In approximately 50 % of cells NO and SNAP inhibited the amplitude of Iswell by about 45 % in a voltage-independent manner. Iswell was also inhibited by an inhibitor of NO-sensitive guanylate cyclase (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and by KT5823, an inhibitor of cGMP-dependent protein kinase. 3. In other cells both NO and SNAP enhanced Iswell by about 40 % in a voltage-independent manner. A similar increase was produced by application of the cell-permeable cGMP analogue 8-bromo-guanosine 3', 5'-cyclic monophosphate (8-Br-cGMP). However, 8-Br-cGMP had no effect on current amplitude in cells pre-treated with KT5823. In contrast 8-Br-cGMP increased the amplitude of Iswell in cells which had been pre-treated with ODQ. 4. SNAP also modulated Iswell recorded in the conventional whole-cell configuration with internal solutions containing 10 mM EGTA to rule out any contribution from Ca2+-activated Cl- currents. 5. These data suggest that the amplitude of Iswell can be enhanced by NO via a cGMP-dependent phosphorylation and inhibited by NO in a cGMP-independent manner.