| Abstract: | Abstract: The main aim of the present investigation was to study systematically the passive and stimulation-evoked release of 3H-5-hydroxytryptamine (3H-5-HT) from rabbit isolated aorta. This was accomplished by preloading rings of aorta with 3H-5-HT (10–6M) and then monitoring by fractional collection the basal 3H-outflow and stimulation-evoked 3H-overflow. The basal 3H-outflow from aorta preloaded with 10–6M of either 3H-5-HT or (-)-3H-noradrenaline (3H-NA) leveled off about 100 min. after the onset of wash-out and remained almost constant thereafter (100–240 min.). The basal 3H-outflow from tissue preloaded with 3H-5-HT was almost 3-fold higher (70–240 min.) than that seen after preloading with 3H-NA. Cocaine (3x10–5M) did not alter the basal 3H-outflow (15–240 min.) from tissue preloaded with 3H-5-HT, while pargyline (5X10–4M) decreased it by about 66% (100–240 min.). Electrical-field stimulation (S1S7, 200 mA, 600 pulses, 0.5 msec, 3 Hz) were applied to the tissue. The initial stimulation-evoked 3H-overflow from aorta preloaded with 3H-5-HT was higher than the subsequent ones (S1-S7: 100, 35, 35, 35, 35, 37, and 40%). Similar results to these were obtained with tissues preloaded with 3H-NA. The stimulation (S1-S7; 200 mA; 600 pulses, 0.5 msec, 3 Hz)-evoked 3H-overflow increased in an apparent linear manner with the amount of current used (50–200 mA). This was also the case for number of pulses (100–900) in the stimulus. The stimulation-evoked 3H-overflow depended in part on the stimulation frequency: unchanged at 2–4 Hz; small increase at 8 Hz; and a 15-fold increase at 16 Hz relative to 2 Hz. Tetrodotoxin (10–6M) decreased the stimulation-evoked 3H-overflow from aorta preloaded with 3H-5-HT (S2-S6) by about 60%, while S1 was not affected. The inhibitory effect of tetrodotoxin was fully reversed by washing the aorta with drug-free salt solution. Omission of Ca2+ from the salt solution reduced the stimulation 3H-overflow by 47–57% (S2-S6) while S1 was unaffected. 6-Hydroxydopamine markedly increased the stimulation-evoked 3H-overflow from 3H-5-HT preloaded rings (180–323% of control). Pargyline (5x10–4M), cocaine (3x10–5M) and removal of endothelium did not alter the stimulation-evoked 3H-overflow evoked by stimulation (S1-S6) of aorta preloaded with 3H-5-HT. It is concluded that 3H-5-HT can be released by electrical-field stimulation as a “false transmitter'’from rabbit isolated aorta. Most of the release is probably of neuronal origin. However, some of the stimulation-evoked 3H-overflow is derived from extraneuronal sites. |
