Non-cholinergic inotropic responses evoked by electric field stimulation in the isolated rat urinary bladder. Possible participation of substance P

The present study explores whether a peptide, such as substance P (SP), has some role subserving the atropine-resistant component of electrically-evoked contractions,in isolated rat urinary bladders. The electric field stimulation (EFS) employed herein,consisted in square wave pulses of 5 Hz, 50 ms duration and supramaximal voltage (40 V), applied for 10 sec, every 3 min and conducted to the tissue via a pair of platinum ring electrodes, surounding the isolated preparations. In order to assess whether electric stimuli, induced urinary bladder inotropism through the activation of nerve structures, degeneration of intramural nerve elements was attempted by cooling the tissue (48 h at 4-5°C). After such procedure, 80-90% inhibition of responses to EFS, was detected. Moreover, tetrodotoxin, at 10^-6 M, evoked similar effects than cooling. Atropine, at 10^-6 M, failed to produce a significant decrement of contractile responses, whereas at 10^-5 M, the electrically-induced inotropism declined around 40% in comparison with controls. In another set od experiments, atropinized urinary bladders (atropine at 10^t̄5 M) were exposed to capsaicin (5 × 10^-6 M) and this coincided with decreased (-43%) responses to EFS. Next, SP, at 10^-9 M, was added to the medium containing capsaicin and complete restoration of full contractile responses to EFS, was observed. In as much as it has been proposed that capsaicin releases SP from sensory nerve fibers and since our experiments show that SP restored the inotropism elicited by electric stimuli on capsaicin-exposed preparations, it is suggested that SP could be involved, at least in part, in the non-cholinergic, EFS-evoked, contractile responses of isolated rat urinary bladders.