Effects of noradrenaline and acetylcholine on electro‐mechanical properties of the guinea‐pig portal vein

Baron

et al. 1991

I The effects of noradrenaline (NA) were studied on vascular smooth muscle cells isolated from rat portal vein. 2 Two types of single-Ca2+ channel currents with conductances of 17pS and 8pS were obtained in cell-attached configuration. Bath application of NA increased the open probability of both channels during depolarizing pulses without a change of background membrane conductance. However, NA did not open Ca2 + channels when the membrane patch potential was held at -50mV, which is about the resting potential in physiological conditions. 3 In the whole-cell configuration, studies of voltage-dependent Ca2+ channel currents showed that the peak conductance curve was not shifted to more negative potentials by NA.4 Measurements of internal Ca2+-concentration ([Ca2+]) with Indo-1 indicated that NA increased [Ca2+] at a holding potential of -50 mV and evoked a Ca2 '-activated Cl-current. These effects were blocked when heparin was included in the pipette solution. 5 A Cl -channel blocker without effect on Ca2 + channels (anthracene-9-carboxylic acid) inhibited the contractions of portal vein strips induced by NA in a manner similar to that produced by a Ca2 + channel inhibitor (isradipine). The NA-induced contraction was completely suppressed in the presence of ryanodine which depletes intracellular Ca2 + stores. 6 The present study suggests that activation of Cl -channels by Ca2+ release produces a membrane depolarization which is a prerequisite for enhanced opening of voltage-dependent Ca2+ channels in response to NA in venous smooth muscle.

Section: Introductionmentioning

confidence: 99%

Ca²⁺ channel activation and membrane depolarization mediated by Cl⁻ channels in response to noradrenaline in vascular myocytes

Pacaud

Baron

et al. 1991

“…It is well known that caffeine displaces calcium from intracellular stores in portal vein smooth muscle (Nanjo, 1984) and we studied the effects of bath-applied caffeine in caesium-containing solution.…”

Section: Calcium-dependence Of the Noradrenaline-induced Inward Currentmentioning

confidence: 99%

“…In the present study we have investigated the ionic mechanisms underlying the depolarization to noradrenaline (Nanjo, 1984) in cultured single cells isolated from rat portal vein using whole-cell recordings with patch-pipettes.…”

Section: Introductionmentioning

confidence: 99%

Noradrenaline activates a calcium‐activated chloride conductance and increases the voltage‐dependent calcium current in cultured single cells of rat portal vein

Pacaud

Mironneau

et al. 1989

1 Membrane responses were recorded by a patch pipette technique in cultured cells isolated from rat portal vein. Using the whole-cell mode, pressure ejections of noradrenaline evoked depolarization (current clamp) and inward current (voltage clamp) at membrane potentials of -60 to -70 mV. The noradrenaline-induced response was reversibly blocked by prazosin indicating that the response was mediated by a1-adrenoceptors.2 The ionic mechanism of the noradrenaline-induced inward current was investigated in potassium-free caesium-containing solutions. Alteration of the chloride equilibrium potential produced similar changes in the reversal potential of the noradrenaline-induced current, indicating that noradrenaline opened chloride-selective channels. There was no evidence implicating sodium or calcium as the charge-carrying ion. 3 Caffeine applied in the bathing solution also induced a transient increase in chloride conductance but the noradrenaline-induced response was lost after application of caffeine. This is interpreted to mean that the increase in chloride conductance induced by noradrenaline and caffeine can occur as a consequence of a rise in intracellular calcium concentration depending on release of calcium from the same intracellular stores. 4 In the presence of caffeine, noradrenaline increased both the voltage-dependent calcium and chloride membrane conductances during application of repetitive depolarizing pulses. It is concluded that in isolated cells of the rat portal vein the depolarization in response to noradrenaline is mediated by an increase in chloride conductance depending on both the calcium release from intracellular stores and the increase of the voltage-dependent calcium current.

“…In Ca2"-free, 0.5 mM EGTA-containing solution, the 60 mM K+-induced contraction ceased within several seconds. In contrast, noradrenaline-and acetylcholine-induced contractions decreased in amplitude but persisted longer (Nanjo, 1984;Dacquet et al, 1987). In the following experiments, the filling of the internal calcium store was carried out on quiescent preparations (in the presence of 50 nM nifedipine) for Omin in 2.1 mM Ca2"-containing solutions.…”

Section: Electrical Activitymentioning

confidence: 99%

“…Spironolactone was a gift from Searle (Paris). Spironolactone and nifedipine were prepared as a 10-' M stock solution in dimethylsulphoxide (DMSO, Sigma) and diluted to the final concentra- Application of a 60mM K' solution induced a phasic peak contraction followed by a sustained plateau the amplitude of which was dependent on the external calcium concentration (Gabella, 1968;Nanjo, 1984). Spironolactone was then added for its equilibration time either before the phasic potassium contraction or during the plateau of the potassium contraction.…”

Section: Electrical Activitymentioning

confidence: 99%

Spironolactone inhibition of contraction and calcium channels in rat portal vein

Dacquet

Mironneau

et al. 1987

1 The effects of spironolactone have been studied on the mechanical activity of rat portal vein strips and the calcium channel currents of isolated cells using the patch clamp technique (whole-cell configuration). 2 Spironolactone (50 nM to 0.1 mM) depressed both K+-induced and twitch contractions within 5-6 min. This inhibitory effect was overcome by elevating the calcium concentration in the perfusing solution.3 Spironolactone (60pM) depressed the transient contractions induced in a Ca2"-free, EGTAcontaining solution by either acetylcholine (0.1 mM) or noradrenaline (10pM). The effect of spironolactone was dependent on a reduction in the filling of the internal calcium store. 4 Rapidly inactivating calcium channel current was maintained in the presence of spironolactone (60 PM), while slowly inactivating calcium channel current was blocked in a concentration-dependent manner. Half-inhibition of slow calcium channel current was obtained at concentrations between 5-7 tM.5 Administration of spironolactone (10 pM) at rest reduced calcium channel current by about 70% (tonic inhibition). Repetitive depolarizations (300 ms long pulses to zero mV, applied between 0.05 and 0.5 Hz) had no further inhibitory effect on the inward current (absence of use-dependence). 6 When cells were held at depolarized membrane potentials at which slow calcium current was inactivated by about 80%, the inhibitory effect of spironolactone (10 EuM) was similar to that obtained with cells normally polarized. Spironolactone (1O JiM) had no effect on the voltage-dependence of inactivation of the calcium channel current. 7 Our results suggest that spironolactone acts primarily on the plasma membrane by depressing inward current through slow calcium channels. This effect may be explained by a preferential binding of the drug to the resting state of the slow calcium channel. In addition, spironolactone may depress contractions dependent on the release of calcium from the sarcoplasmic reticulum.