The rat ureter has a distinctive action potential, with an initial fast, single spike, followed by a temperature-sensitive plateau phase (Burdyga & Wray, 2002). Ca2+-induced Ca2+ release is absent, unlike in guinea-pig ureter, although agonists can increase [Ca2+] (Burdyga et al. 1995). As the ionic currents in rat ureter have not been characterised, we determined them in order to better understand the above processes.
Wistar rats (250Ð350 g) were killed by cervical dislocation and their ureters removed. Single cells were isolated by enzymatic digestion and held at -80 mV using the whole-cell patch-clamp technique. Cells were depolarised in either caesium or potassium pipette solution to reveal inward or outward currents, respectively. All experiments were carried out at room temperature and all values represent means ± S.E.M.; n is the number of cells.
Stepping to depolarizing potentials produced a voltage-dependent slow non-inactivating current. The peak amplitude of this current was 1323 ± 180 pA (n = 14) and it was inhibited by 1 mM tetraethylammonium (TEA; 35 ± 7 % of the control; n = 4). At a concentration of 10 mM TEA, 20 % of the current was still present (n = 2). In some cells a voltage-sensitive fast transient outward current similar to the A-current was seen. This current had a peak amplitude of 424 ± 60 pA, and was inhibited by 1 mM 4-aminopyridine (49 ± 4 % of control; n = 3). A calcium-dependent inward tail current was observed which was abolished by cadmium chloride. When caesium was present in the pipette, depolarisation produced voltage-dependent inward currents (peak 61 ± 8 pA; n = 14) which had a bell-shaped currentÐvoltage relationship, were inhibited by 1 mM cadmium chloride and 1 µM nifedipine, and were enhanced by 1 mM Bay K 8644 (232 ± 43 % of the control; n = 14).
These results suggest that several K currents exist in these cells, a TEA-sensitive calcium-activated K current, a TEA-insensitive delayed rectifier and a fast transient outward current that was inhibited by 4-aminopyridine. The bell-shaped currentÐvoltage relationship and the ability of cadmium chloride and nifedipine to abolish the inward current suggests that this current is via L-type voltage-operated calcium channels. The calcium-dependent inward tail current may be due to a calcium-activated chloride current. Further experiments using more specific inhibitors and elevated temperature are in progress to clarify these findings.
We gratefully acknowledge the financial support of The Wellcome Trust.
All procedures accord with current UK legislation.