Modulators of internal Ca2+ stores and the spontaneous electrical and contractile activity of the guinea-pig renal pelvis

Abstract

1. The role of internal Ca2+ stores in the generation of the rhythmic electrical and contractile activity in the guinea-pig proximal renal pelvis was examined using intracellular microelectrode and muscle tension recording techniques. 2. Ryanodine (30 μM) transiently increased contraction amplitude, while caffeine (0.5-3 mM) reduced contraction amplitude and frequency. Contractility was also reduced by 2-aminoethoxydiphenylborate (2-APB 60 μM), xestospongin C (1 μM), U73122 (5 μM) and neomycin (4 mm), blockers of IP3-dependent release from Ca2+ stores. 3. 60 mM K+ saline-evoked contractions were reduced by caffeine (1 mm), U73122 (5 μM) and neomycin (4 mm), but little affected by ryanodine or 2-APB (60 μM). 4. Spontaneous action potentials consisting of an initial spike followed by a long plateau were recorded (frequency 8.6±1.0 min−1) in small urothelium-denuded strips of proximal renal pelvis. 5. Action potential discharge was blocked in 75 and 35% of cells by 2-APB (60 μM) and caffeine (1 mM), respectively. In the remaining cells, only a truncation of the plateau phase was observed. 6. Cyclopiazonic acid (CPA 10 μM for 10-180 min), blocker of CaATPase, transiently increased contraction frequency and amplitude. Action potential durations were increased 3.6 fold. Contraction amplitude and frequency slowly declined during a prolonged (> 60 min) CPA exposure. 7. We conclude that the action potential in caffeine-sensitive cells and the shoulder component of caffeine-insensitive action potential arise from the entry of Ca2+ through Ca2+ channels. The inhibitory actions of modulators of internal Ca2+ release were partially explained by a blockade of Ca2+ entry.