Mechanism of presynaptic actions of adenosine and acetylcholine on noradrenaline release in the guinea-pig heart

Abstract

Isolated heart of the guinea-pig was stimulated transmurally (1 Hz), or by excess [K +] to evoke release of [ 3H]noradrenaline. The interactions of tetraethylammonium, acetylcholine and adenosine were investigated on [ 3H]noradrenaline overflow at different [Ca 2+] concentrations. One mM tetraethylammonium doubled [ 3H]noradrenaline overflow, and at 30 mM the overflow was facilitated by about 50-fold. The facilitatory effect of 30 mM tetraethylammonium gradually decreased with reduction in [Ca 2+] concentration. In 0.1 mM [Ca 2+] overflow of [ 3H]noradrenaline was completely blocked. However, addition of 30 mM tetraethylammonium to 0.1 mM [Ca 2+]-Krebs solution greatly facilitated the overflow. 35 mM K-induced [ 3H]noradrenaline overflow was facilitated by about 5-fold with 20 mM tetraethylammonium, and the facilitatory effect was blocked by 0.37 μM tetrodotoxin; 75 mM K-induced overflow was facilitated by only 1.5-fold by tetraethylammonium. Electrically induced overflow of [ 3H]noradrenaline was significantly reduced (30%) by 0.13 μM acetylcholine, and the effect increased up to about 75% with higher concentrations of acetylcholine (1.3 μM). 1.3 μM acetylcholine reduced 35 mM K-induced overflow by 30% and had no effect on 75 mM K-induced overflow. 0.37 μM tetrodotoxin had an almost similar effect on K-induced overflow. Ten-times higher concentrations of acetylcholine were needed to obtain over 70% inhibition of overflow produced by low or high K. The inhibitory effect of acetylcholine on [ 3H]noradrenaline overflow by 1 Hz was antagonized by increasing concentrations of tetraethylammonium (3 to 30 mM) in 2.5 mM Ca. The antagonism between acetylcholine and tetraethylammonium persisted even when Ca was lowered to 0.02 mM. However, if the overflow was enhanced by other means (high [Ca 2+] plus phentolamine) to the same extent as with tetraethylammonium and low [Ca 2+], then acetylcholine blocked overflow by 75%. 3.7 μM adenosine reduced [ 3H]noradrenaline overflow by about 54%, and this effect was completely prevented by 30 mM tetraethylammonium, either in 2.5 or 0.02 mM Ca. It is proposed that acetylcholine and adenosine interfere with nerve stimulation-evoked release of [ 3H]noradrenaline, presumably by altering electrical properties (i.e. conduction, resting membrane potential, duration of nerve action potential, etc.) of cardiac sympathetic nerve terminals. This primary action would subsequently lead to a reduction in the availability of [Ca 2+] for the release process.