Responses for smooth-muscle strips from the ileo-caecal region to transmural electrical stimulation

Under isometric conditions simultaneous recordings are made of the contractile activity of smooth-muscle strips (2/20 mm) isolated from the terminal ileum (T1), the ileo-caecal sphincter (ICS) and the caecum (C) of cats. Transmural electrical stimulation is applied (0.1-0.5 ms. 2-15 Hz at supramaximal current) for 20-60 s. ICS manifests tonic changes (1st type of activity) and sometimes phasic contractions (second type of activity); phasic contractions are recorded in T1 and C. Switching on of the stimulation causes relaxation (on-response) of ICS, with maximum values in the 5-9 Hz interval. Two types of off-response are observed when the stimulation is switched off: in the strips of the first type of off-response is characterized by tonic contraction only, while in the strips of the second type there is high-amplitude contraction followed by a rise in the tone on which phasic contractions are superimposed. Inhibition of the phasic contraction and a slight decrease of the tone characterize the on-response resembles that of the ICS strips of the second type. In the C strips there is an evoked high-amplitude phasic contraction when the stimulation is switched on. The responses of ICS, T1 and C to transmural electrical stimulation are nerve-transmitted, because they are blocked by TTX (10-6 g/l). On the basis of the analysis with atropine (10-6 M), phentolamine (10-5M) and propranolol (10-5M) it is assumed that nonadrenergic inhibitory mechanisms are involved in the on-response of ICS to transmural electrical stimulation, while predominantly cholinergic and adrenergic mechanisms of regulation are manifested in the off-response of ICS and in the responses of T1 and C.