ABSTRACT
A homogeneous population of isolated cells is a simple preparation available for measurement of the action of an agonist in altering cell length. This preparation has the advantage of eliminating diffusion barriers, allowing measurement of the direct action of the agonist unaffected by other control systems and providing the opportunity for studying several aspects of cell function such as contractility, membrane potential and intracellular calcium. Highly selective analogues for the subtypes of neurokinin receptors were used to study responses of the canine antrum, pylorus and duodenum in vivo and in vitro. The NK-1 excitatory responses were suggested to be present on both neural and muscle sites in all three in vivo and only muscle sites in vitro. The discovery that the opiates produce motor activity by inhibiting release of vasoactive intestinal peptide (VIP) in the canine small intestine provided an explanation of some of these discrepancies. In this experimental circumstance, opiates could both depress VIP output and induce motor activity.
