Frequency dependence of muscarinic facilitation of transmitter release in urinary bladder strips from neurally intact or chronic spinal cord transected rats

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Abstract
1. Electrical stimulation evoked release of 3H-noradrenaline (NA) and 14C-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 (SCT) 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 strips. 3. Atropine (1 microM) depressed ACh release in NI bladder strips at high frequency stimulation (10 and 40 Hz) but not at low frequency stimulation (2-5 Hz). However, in SCT bladders, atropine depressed ACh release both at low and high frequencies of stimulation, indicating that muscarinic facilitation occurs at lower frequencies. 4. Atropine depressed 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 enhanced as the frequency of stimulation was increased from 1 to 40 Hz (80 shocks). The frequency response curve was shifted to the left in SCT bladders. Atropine blocked the neurally evoked contractions in SCT bladder strips to a greater extent than the contractions in NI strips indicating a cholinergic dominance in the SCT bladders. 6. Maximal contractile force of SCT bladder strips evoked by neural stimulation at 20 Hz 10 shocks and 80 shocks was significantly lower than that of NI bladder strips, whereas the release of ACh was significantly higher in SCT than NI bladders indicating a postjunctional defect in the SCT preparations. 7. It is suggested that presynaptic muscarinic facilitatory mechanisms are upregulated in the cholinergic and adrenergic nerve terminals 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 enhancement of transmitter release at bladder postganglionic nerve terminals.

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