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Notizen: Summary Non-cholinergic, non-adrenergic responses to nerve stimulation recorded from smooth muscles of the guinea-pig duodenum, jejunum, proximal and terminal ileum were investigated in an attempt to characterize these responses. In the presence of atropine (1–2 μmol · l−1) and guanethidine (10 μmol · l−1) coaxial stimulation induced in all regions of the guinea-pig small intestine an initial relaxation (primary relaxation) upon which contraction (primary contraction) appeared, followed by rebound contraction. Noradrenaline decreased the cholinergic smooth muscle twitches, predominantly at low stimulation frequencies, and had a similar effect on the non-cholinergic, non-adrenergic primary relaxation, primary and rebound contractions. ATP decreased the smooth muscle twitches; however, this agent had only a transient influence on the non-cholinergic, non-adrenergic responses of muscle (tension and membrane potential) to single stimuli. With higher stimulus frequencies ATP increased the primary relaxation and decreased the contraction phases. ATP also inhibited the post-tetanic inhibition induced by non-cholinergic, non-adrenergic nerve stimulation. In most of the muscle cells of the guinea-pig proximal and terminal ileum the non-cholinergic, non-adrenergic nerve stimulation generated i.j.p.s., while about 15–20% of the cells responded with e.j.p.s. During long-lasting stimulation (10s) the i.j.p.s. were sometimes “interrupted” by action potentials or by a gradual depolarization of the membrane. The i.j.p.s. were followed by a marked rebound depolarization accompanying the action potentials. Those cells which generated i.j.p.s. in response to field stimulation, were depolarized by ATP, while those cells, which generated e.j.p.s, were hyperpolarized by ATP. A reduction in the concentration of extracellular sodium chloride decreased both the primary and rebound contractions; the primary contraction was, however, more sensitive than was the rebound contraction. Theophylline increased the primary and rebound contractions with no marked influence on the primary relaxation, lowered the action potential threshold, increased the rebound depolarization and did not markedly influence the i.j.p.s. Quinidine enhanced the primary relaxation and inhibited the primary contraction in a concentration-dependent manner. Inhibition of the rebound contraction by quinidine was slight (less than 50%). The present results demonstrate that primary relaxation, primary and rebound contractions are associated with i.j.p.s and e.j.p.s., and rebound depolarization with action potentials, respectively; they are typical responses of various regions of the guinea-pig small intestine to activation of inhibitory and excitatory non-cholinergic, non-adrenergic nerves. The P1 and P2 receptors, proposed by Burnstock (1975), probably do not mediate the non-cholinergic, non-adrenergic postsynaptic responses of the guinea-pig small intestine. A possible physiological function of ATP as a mediator of non-cholinergic, non-adrenergic nerves of the guinea-pig small intestine is discussed.