Effects of calcium entry blockers on calcium‐dependent contractions of rat portal vein

Rakotoarisoa

et al. 1989

Self Cite

Specific binding of the calcium‐antagonist dihydropyridine derivative, (+)−[3H]‐PN 200–110 (isradipine), to cell membranes of equine portal vein smooth muscle was compared with binding to intact strips isolated from rat portal veins. Specific binding to vascular smooth muscle membranes was of high affinity, saturable and reversible. The dissociation constant obtained from association and dissociation kinetics of (+)−[3H]‐PN 200–110 was similar to that obtained from equilibrium binding and competition experiments. Specific binding of (+)−[3H]‐PN 200–110 was completely displaced by unlabelled dihydropyridines. Among other calcium antagonists, D888 and (+)−cis‐diltiazem partially inhibited the binding at 25°C. At 37°C, only (+)−cis‐diltiazem stimulated the binding. LaCl3, CdCl2, NiCl2, CoCl2 had inhibitory effects, whereas KCl and NaCl had no effect. When intact strips of portal vein were incubated in high external potassium concentrations for 30 min, the Kd was lowered to 0.04 ± 0.01 nm from the control value of 0.14 + 0.02 nm (n = 5), thereby indicating that (+)−[3H]‐PN 200–110 bound to voltage‐dependent calcium channels, with a higher affinity, in the depolarized state. When external Ca2+ was removed or substituted with Ba2+ or Sr2+, Kd values increased suggesting that the dihydropyridine binding to intact strips was modulated by binding of Ca2+ ions to voltage‐dependent calcium channels.

Section: Discussionmentioning

confidence: 95%

“…state of the channel. (2) In contraction experiments, the voltage-dependence of calcium channel blockade is indicated by a shift to the left of the doseinhibition curves measured in physiological solution or in K+-rich depolarizing solution (Dacquet et al, 1987). (3) The third method is to use radioligand binding techniques.…”

Section: Introductionmentioning

confidence: 99%

(+)−[³H]‐PN 200–110 binding to cell membranes and intact strips of portal vein smooth muscle: characterization and modulation by membrane potential and divalent cations

Dacquet

Rakotoarisoa

et al. 1989

Self Cite

“…Contractions of portal vein longitudinal strips were measured with an isometric force transducer (Akers 801) as previously described (Dacquet et al, 1987 (Hamill et al, 1981). Whole-cell Cl-currents were recorded in response to 10-5M NA applied from a glass pipette with a pressure ejector.…”

Section: Contraction Measurementsmentioning

confidence: 99%

“…In portal vein smooth muscle, stimulation of a1-adrenoceptors produces contraction that involves release of Ca2+ from intracellular stores and Ca2 + influx through voltage-dependent Ca2 + channels (Mironneau & Gargouil, 1979;Dacquet et al, 1987). The NAinduced contraction is associated with a depolarization and an increase in membrane conductance (Takata, 1980;Nanjo, 1984).…”

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

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.

“…Stimulation of al-adrenoceptors in portal vein smooth muscle produces contraction that can be dependent on an increase in both calcium influx through voltage-dependent channels and release of calcium from intracellular stores (Mironneau & Gargouil, 1979;Dacquet et al, 1987;Pacaud et al, 1987). However, there are recent results concerning the ionic mechanisms underlying the electrophysiological early response to noradrenaline in veins which indicate that the noradrenaline-induced depolarization is mainly mediated by an increase in membrane chloride conductance (Byrne & Large, 1988a, b;Van Helden, 1988).…”

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

Self Cite

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.