Effects of Cl channel blockers on Ca‐activated chloride and potassium currents in smooth muscle cells from rabbit portal vein

Abstract

1 The effects of some chloride channel antagonists were studied on the calcium-activated chloride current (ICl(ca)) in smooth muscle cells from the rabbit portal vein with the perforated patch technique.2 4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS) and 4,4'-diisothiocyanato-stilbene-2,2'-disulphonic acid (DIDS) reduced the amplitude of spontaneous transient inward currents (STICs, calcium-activated 5 DIDS and SITS decreased the amplitude of IC1(ca) evoked by noradrenaline and caffeine less potently than STICs with IC50 values of 7.5 x 10-4 M and 1.8 x 10-3 M, respectively. 6 DIDS and SITS increased the calcium-activated potassium current (IK(C)) evoked by noradrenaline and caffeine by 3-6 fold. 7 Anthracene-9-carboxylic acid (A-9-C) inhibited STICs in a voltage-dependent fashion and was about 3 fold more active at + 50 mV than at -50 mV. A-9-C increased STIC r and this effect was enhanced by depolarization.8 A-9-C also inhibited caffeine-evoked IC1(ca) but less potently than STICs and also increased the evoked IK(ca) without altering spontaneous IK(Ca). 9 The results from the present work are compared with the pharmacology of other chloride conductances and the mechanism of action of the chloride channel antagonists in vascular smooth muscle is discussed.

Action of niflumic acid on evoked and spontaneous calcium‐activated chloride and potassium currents in smooth muscle cells from rabbit portal vein

“…Since niflumic acid increased the noradrenaline-evoked IK(Ca) without altering STOCs (spontaneous calcium-activated potassium currents) it is possible that this compound increases the evoked release of calcium from the intracellular store without altering the spontaneous release of calcium. It was observed previously that DIDS, SITS and A-9-C increased the evoked IvCac) (Hogg et, al., 1994) and it may be a general property of chloride channel blocking agents to enhance the amount of calcium released from the intracellular store.…”

“…Voltage-dependent effect of niflumic acid Previously we had demonstrated that A-9-C reduced STICs in a voltage-dependent manner whereas the inhibitory action of DIDS and SITS was not influenced by membrane potential (Hogg et al, 1994). We have carried out similar experiments with niflumic acid in potassium-free bathing and pipette solutions to eliminate STOCs which become prominent at potentials positive to -50 mV.…”

“…It has been demonstrated previously that DIDS, SITS and A-9-C were less potent in inhibiting evoked chloride currents compared to STICs (Hogg et al, 1994). Consequently we investigated the effect of niflumic acid on noradrenaline-and caffeine-evoked IcQ(ca) in potassium-free conditions at a holding potential of -50 mV.…”

“…carboxylic acid (A-9-C) inhibit evoked Icc) in rat portal vein (Baron et al, 1991). Also it was shown in rabbit portal vein that these compounds were more potent against spontaneous transient inward currents (STICs, chloride currents activated by spontaneous release of calcium from caffeinesensitive intracellular stores) than against Ic~ca) elicited by noradrenaline and caffeine (Hogg et al, 1994). It was found that all 3 compounds had low potency as the concentration to inhibit IcQ(c,,) by 50% was greater than 10-4 M i.e.…”

“…Since it has been shown that an anion and a cation conductance increase is responsible for the noradrenaline-induced depolarization recorded with microelectrodes in isolated cells of the rabbit portal vein (Amedee & Large, 1989), it is possible that ICk,,Ca) and It,, may have important roles in producing depolarization and contraction in vascular smooth muscle. Also it is relevant that Icj(ca) has now been observed in several types of smooth muscle and can be activated by various pharmacological agents (see Introduction of Hogg et al, 1994). Therefore it would be interesting to assess the contribution of IQ(ca) and Icat to agonist-induced depolarization in smooth muscle.…”