The effects of ATP related compounds on the electrical activity of the rat portal vein

Abstract

1. The effects of purine compounds, ATP, ADP, adenosine and AMP-PNP on electrical properties of the rat portal vein were investigated by the microelectrode method. 2. The membrane potential was −44.2 ± 2.1 mV ( n = 30). AMP-PNP, ATP, AMP and adenosine increased the spike frequency and the relative potencies to enhance the spike generation and the depolarization of the membrane were in order to AMP-PNP > ATP > AMP > adenosine. The depolarization of the membrane was observed in concentration of 10 −7 M AMP-PNP, 10 −6 M ATP or 10 −5 M AMP. However, no depolarization was observed by treatment with 10 −4 M adenosine but this concentration of adenosine increased the spike frequency. 3. The ATP induced depolarization was consistently observed in high [K] 0 and low [K] 0, high [Ca] 0 and low [Ca] 0, as well as in Krebs solution but not in low [Na] 0. The membrane was transiently hyperpolarized by ATP in the presence of low [Na] 0. 4. The current-voltage relationship was obtained before and during application of ATP, and ATP reduced the membrane resistance at any given membrane potential level. Therefore, it was concluded on the ATP action on the membrane potential that the depolarization of the membrane is mainly due to an increase in the Na-conductance and in part to an increase in the K-conductance of the membrane. 5. Excitatory actions induced by purine compounds were not modified by adrenergic α- and β-antagonists. 6. Up to the concentration of 10 −4 M, adenosine did not modify the membrane potential in Krebs solution, and also in low and high [Ca] 0, low and high [K] 0 and in low [Na] 0. Furthermore, the noradrenaline induced depolarization was not affected by adenosine. 7. The actions of purine compounds on the muscle membrane were discussed in relation to mechanical responses.