ATPo but not cAMPi activates a chloride conductance in mouse ventricular myocytes

Abstract

Objective: The aim was to test for the presence of cAMPi activated and ATPo activated chloride conductances (ICl(cAMP) and ICl(ATP)) in mouse ventricular myocytes.

Methods: Membrane currents were measured in enzymatically dissociated ventricular myocytes using the whole cell configuration of the patch clamp technique.

Results: After inhibiting the activation of sodium, calcium, and potassium currents, increasing [cAMP]i with isoprenaline (1 microM), forskolin (10 microM), or isobutylmethylxanthine (500 microM) had no effect on residual membrane current but extracellular ATP (100 microM) elicited a time independent background conductance. The ATP-activated conductance was observed during voltage steps ranging from +40 to -110 mV from a holding potential of -30 mV within 1 min after adding ATP to the bath. The reversal potential for the ATP activated conductance varied with changes in the chloride gradient and was within several mV of the predicted Nernst potential for chloride. Partially substituting external chloride with aspartate attenuated the ATP activated currents and shifted the reversal potential to values close to those predicted for ECl, suggesting the chloride selective nature of the conductance. The poorly hydrolysable ATP analogue ATP gamma S (100 microM) also elicited the chloride conductance, suggesting purinoceptor linked activation rather than a mechanism dependent on hydrolysis of ATP by ectoATPases. The conductance may be activated following P2 and not P1 purinoceptor stimulation, since the P1 purinoceptor agonists adenosine (500 microM) and adenosine monophosphate (500 microM) had no effect in seven and four cells, respectively. Activation of ICl(ATP) is not likely to be dependent on increased [Ca2+]i since ATP activated the current in cells dialysed with 10 mM EGTA or 20 mM BAPTA.

Conclusions: (1) cAMPi does not activate a chloride conductance in mouse ventricular myocytes; (2) ATPo does activate a chloride conductance, through stimulation of a P2 purinoceptor; and (3) Ca2+i and cAMPi are not involved in activation of ICl(ATP). Also, macroscopic ICl(ATP) from mouse ventricle and ICl(cAMP) from other species appear to be indistinguishable in terms of time independence and rectification properties that depend on the concentration and permeability of intracellular anions. ICl(ATP) could be of physiological and pathophysiological significance considering the ability of chloride channels to modulate cardiac electrical activity and the fact that ATP may be an important neuromediator in the heart.