FREQUENCY-DEPENDENCE OF MUSCARINIC FACILITATION OF TRANSMITTER RELEASE IN URINARY-BLADDER STRIPS FROM NEURALLY INTACT OR CHRONIC SPINAL-CORD TRANSECTED RATS

1 Electrical stimulation evoked release of H-3-noradrenaline (NA) and C-14-acetylcholine (ACh), as well as neurally evoked contractions were measured at Various (1-40 Hz, 100 shocks) stimulation frequencies in bladder strips from neurally intact (NI) and spinal cord transected (S CT) rats. 2 The frequency response curves for ACh and NA release were shifted to the left in SCT bladder strips as compared to NI bladder st rips. 3 Atropine (1 mu M) depressed ACh release in NI bladder strips a t high frequency stimulation (10 and 40 Hz) but not at low frequency s timulation (2-5 Hz). However, in SCT bladders, atropine depressed ACh release both at low and high frequencies of stimulation, indicating th at muscarinic facilitation occurs at lower frequencies. 4 Atropine dep ressed the release of NA in NI bladders at only 40 Hz stimulation, but depressed release at all frequencies in SCT bladders. 5 The amplitude of neurally evoked contractions of bladder strips from NI rats was en hanced as the frequency of stimulation was increased from 1 to 40 Hz ( 80 shocks). The frequency response curve was shifted to the left in SC T bladders. Atropine blocked the neurally evoked contractions in SCT b ladder strips to a greater extent than the contractions in NI strips i ndicating a cholinergic dominance in the SCT bladders. 6 Maximal contr actile force of SCT bladder strips evoked by neural stimulation at 20 Hz 10 shocks and 80 shocks was significantly lower than that of NI bla dder strips, whereas the release of ACh was significantly higher in SC T than NI bladders indicating a postjunctional defect in the SCT prepa rations. 7 It is suggested that presynaptic muscarinic facilitatory me chanisms are upregulated in the cholinergic and adrenergic nerve termi nals in SCT bladders leading to a larger relative contractile response at lower frequencies of stimulation (2-5 Hz). Thus the hyperreflexic bladder occurring after spinal cord injury may be due in part to an en hancement of transmitter release at bladder postganglionic nerve termi nals.