Activation and blockage of a calcium‐sensitive cation‐selective pathway in the apical membrane of toad urinary bladder.

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Abstract
1. The properties of cation movements through a previously described Ca2+-sensitive oxytocin-stimulated pathway in the apical membrane of the toad urinary bladder were further investigated. 2. In the absence of Ca2+ and other polyvalent cations in the mucosal medium, oxytocin markedly stimulated the flow of current from mucosa to serosa when the major cation in the mucosal solution was any of the following ions: Na+, K+, Rb+, Cs+ or Li+. Analysis of the current noise showed a Lorentzian component associated with the movement of these cations. 3. Ca2+ and other divalent cations in the mucosal solution depressed both the current and the Lorentzian component of the fluctuation spectra. The Michaelis-Menten constants were 2.5, 10 and 58 .mu.mol/l for Ca2+, Sr2+ and Mg2+ respectively. 4. The dihydropyridine Ca2+ channel blockers nitrendipine (10-5 mol/l) and nicardipine (10-6 mol/l) inhibited the Ca2+-sensitive current. 5. Alterations of the mucosal pH showed that the current and the plateau of the Lorentzian component increased by elevating the pH from 6 to 8. The Ca2+-sensitive current was further stimulated by increasing pH to 9. However, this manoeuvre resulted in the disappearance of the Lorentzian component in the noise spectrum. 6. Increasing either mucosal [Na+] or [K+] up to 115 mmol/l did not lead to saturation of the current passing through the Ca2+-sensitive channel. In contrast the amiloride-sensitive channel showed saturating behaviour when mucosal [Na+] was increased; half-maximum current was reached when mucosal [Na+] was about 15 mmol/l. 7. When a Na+-free mucosal solution, prepared with either choline or TEA as major cation, was rapidly replaced by a solution with Na+ (115 mmol/l), the current through the Ca2+-sensitive channel increased rapidly and then remained at a nearly constant level. This behaviour is in contrast with the response of the current through the amiloride-sensitive pathway. After suddenly increasing mucosal Na+ concentration ([Na+]m), the current through this channel first increased rapidly and then declined to values of nearly 50% of the peak about 10 min after the increase in [Na+]m. 8. Current flowed from serosa to mucosa through the Ca2+-sensitive pathway when the serosal side was perfused with a solution with high [K+] and the apical perfusate contained choline as the major monovalent cation. 9. Perfusion with three substances, chlorophenylthio-cyclic AMP, theophylline and forskolin, that increase cyctoplasmic cyclic AMP stimulated current flow through the Ca2+-sensitive pathway.