THE ACTIONS OF ADENOSINE 5'-TRIPHOSPHATE ON GUINEA-PIG INTRACARDIAC NEURONS IN CULTURE

The actions of adenosine 5'-triphosphate (ATP) and related nucleotide and nucleosides on the membrane ion conductances of M and AH type intracardiac neurones cultured from ganglia within the atria and interatrial septum of newborn guinea-pig heart were studied with intracellular current- and voltage-clamp techniques. Approximately 74% (120 out of 161) of AH type cells and 41% (5 out of 12) M cells responded to direct application of ATP (500 μM) onto their soma. In 41% of M and 43% of AH type cells, focal application of ATP (500 μM) evoked rapid depolarization with an increase in conductance which frequently elicited action potential discharge. The underlying inward current had a null potential of -11.2 mV and was reduced in solutions containing low extracellular sodium and calcium but unaffected by reduced chloride-containing solutions. In a further 31% of AH type cells, ATP evoked a multi-component response consisting of an initial depolarization followed by a hyperpolarization and a slow prolonged depolarization. The current underlying the initial depolarization resulted from an increase in conductance and had a null potential of -19.1 mV. The current was increased in low chloride-containing solutions and was only slightly reduced in low sodium- and calcium-containing solutions. The subsequent hyperpolarization and outward current resulted from an increase in membrane conductance and had a null potential of -88.5 mV, which was close to the potassium equilibrium potential in these cells. The slow depolarization and inward current was not associated with change in membrane conductance. In less than 2% of AH cells, ATP evoked a second type of slow depolarization. This was associated with a fall in conductance and had a null potential of -90.7 mV. In 40% of AH cells, adenosine (10-100 μM) inhibited the calcium-sensitive potassium current responsible for the after-hyperpolarization. The action of adenosine was antagonized by the P1-purinoceptor antagonist 8-phenyltheophylline (1-10 μM). The potency order of agonists for all of the ATP-evoked responses, except the slow depolarization associated with a fall in conductance was ATP > ADP with AMP and adenosine being ineffective. Responses to ATP were ony weakly desensitized by α,β-methylene ATP (3 x 10-6 M) and the potency order of analogues was 2-methylthio ATP ≥ ATP > α,β-methylene ATP, indicating the involvement of receptors similar to P(2Y) purinoceptors.