Mechanism of negative feed-back inhibition of norepinephrine release by alpha-adrenergic agonists

The mechanism of alpha-adrenergic receptor-mediated feedback regulation of [3H]norepinephrine release was studied in the guinea-pig heart. The overflow of [3H]norepinephrine evoked by stimulation (1 Hz for 60 s) was inhibited up to 90% in a dose-dependent manner by norepinephrine (58.5 and 177 nM) and epinephrine (54 and 164 nM). These inhibitory effects were antagonized by 20 mM tetraethylammonium. The overflow of [3H]norepinephrine was near normal when the calcium concentration of the perfusion medium was reduced from a control value of 2.5 to 0.25 mM in the presence of 20 mM tetraethylammonium. Norepinephrine exerted its typical inhibitory effect on the overflow induced in low [Ca2+] and high [tetraethylammonium] solution. Similarly, the inhibitory effects of epinephrine became evident in the presence of 20 mM tetraethylammonium and 0.25 mM calcium ions. Another K-channel blocker, 4-aminopyridine, was also effective in antagonizing the presynaptic actions of norepinephrine; the effect was more pronounced at lower than higher concentrations of norepinephrine. However, after reduction of the calcium ion concentration to 0.2 mM in the presence of 1 mM 4-aminopyridine, all concentrations of norepinephrine significantly reduced the overflow of [3H]norepinephrine. We conclude that the alpha-adrenergic agonists directly influence the availability of calcium ions needed for the transmitter release, and that this effect is brought about without altering the electrical properties (i.e. nerve conduction, action potential size, resting membrane potential of the nerves, etc.). This is in contrast to the actions of acetylcholine and adenosine which, in an earlier study, were shown to inhibit [3H]norepinephrine release primarily by modifying the electrical properties of the nerves.