Calcium-activated chloride currents (I_ClCa) contribute to smooth muscle excitation-contraction coupling and are also thought to play an important role in rhythmic electrical activity. This current has been studied extensively in a variety of smooth muscle models but remains to be fully characterised in a murine model, which is important for possible future experiments on animals that have been modified genetically. Therefore the aim of this study was to investigate I_ClCa in the mouse portal vein (mPV), a tissue that has been shown to display the current in preliminary studies (Britton et al. 2002).

Female BALB-c mice (6-8 weeks) were humanely killed by cervical dislocation and the PV was immediately removed and placed in 100 µM Ca²⁺ physiological salt solution. Single cells were dispersed using enzymatic digestion following the removal of fat and connective tissue.

Using the conventional whole cell patch clamp technique and a pipette solution containing 126 mM CsCl, 10 mM Hepes, 4 mM MgCl₂, 5 mM Na₂ATP and 5 mM EGTA, depolarisation from -60 mV evoked an inward current typical of the L-type Ca²⁺ current (I_Ca). In 1.5 mM Ca²⁺ external solution the maximum current was 28 ± 3 pA (mean ± S.E.M., n = 20), which increased to 67 ± 7 pA (n = 19) in 10 mM Ca²⁺ external solution and 100 ± 12 pA (n = 11) in BAY K8644 but was inhibited by nicardipine (5 ± 2 pA, n = 7). The V_0.5 for inactivation of this current was -25 ± 1 mV.

I_ClCa was studied using the perforated patch technique by adding 240 µg ml^-1 of amphotericin to the pipette solution. In 70 % of the 51 cells studied, depolarisation from -60 mV evoked I_Ca followed by a tail current (I_TAIL) upon repolarisation. The amplitude of I_TAIL was extremely variable from cell to cell averaging 120 ± 30 pA in 1.5 mM Ca²⁺ external solution (range from -13 pA to -300 pA, n = 12) and 190 ± 54 pA in 10 mM Ca²⁺ external solution (n = 11). Modulators of I_Ca such as nicardipine and BAY K 8644 abolished and augmented I_TAIL, respectively. I_TAIL was also abolished by membrane rupture leading to intracellular dialysis with EGTA. I_TAIL was inhibited by niflumic acid (mean inhibition was 55 ± 8 % and 73 ± 9 % for 10 µM and 20 µM, respectively) and had a reversal potential (-4 ± 4 mV) close to the theoretical chloride equilibrium potential. Replacement of the external anion with thiocyanate, isethionate and glutamate changed the reversal potential to -46 ± 4, 18 ± 5 and 30 ± 2 mV, respectively (n = 4).

This study shows that mPV myocytes display robust I_ClCa that is activated as a result of Ca²⁺ influx through dihydropyridine-sensitive voltage-dependent calcium channels.

S.N.S is sponsored by a BBSRC/GSK studentship. I.A.G is a Wellcome Trust Research Fellow.