Selective depletion of the acetylcholine and vasoactive intestinal polypeptide of the guinea-pig myenteric plexus by differential mobilization of distinct transmitter pools

The effect of electrical field stimulation on the release of acetylcholine (ACh) and vasoactive intestinal polypeptide (VIP) from superfused strips of myenteric plexus-longitudinal muscle (MPLM) of guinea-pig ileum and on the transmitter content of the tissue was investigated at different frequencies and in the presence and absence of choline hemicholinium-3 and colchicine. Low frequency electrical field stimulation released ACh by more than 4 times the basal release; the simultaneously detected VIP secretion was increased only slightly above the resting level. During high frequency stimulation (50 Hz) the release of VIP was greatly increased (to 5 times the resting release) whereas the release of ACh increased to only 150% of the basal output. When choline was present, the ACh content of the tissue itself was not altered by electrical stimulation indicating a rate of synthesis sufficient to maintain release. It was reduced in a frequency-dependent manner in the absence of exogenous choline or in the presence of 10 μM hemicholinium-3 (an inhibitor of choline uptake) by up to 54% of the original content. A similar but even larger reduction took place in the amount of ACh released. Neither the secretion of VIP nor the tissue VIP content was altered by these treatments. Long-lasting (>60 min) high-frequency (50 Hz) stimulation resulted in the depletion of the VIP pool (by 25%) while the ACh content remained unaltered. The stimulus-induced depletion of the VIP pool was increased when colchicine reduced the amount of VIP released and the VIP content of the stimulated tissue up to 50%; it caused a small hemicholinium-like effect on the release of ACh and on the ACh content of the tissue when the tissue was stimulated for an extended length of time (120 to 180 min) at low frequency (5 Hz) but otherwise had little effect. Depolarization by elevated potassium concentration or by veratridine increased the ACh release by 150-280% and the VIP release by 125-200% of the resting release and was thus equivalent to electrical stimulation at 8-10 Hz. At the end of prolonged chemical depolarization (60 min) the ACh content was reduced to 60-80% and the VIP content to 80-90% of the non-stimulated values respectively. These results are interpreted in terms of the different intracellular dynamics of ACh and VIP and their respective storage vesicles.