Effects of flavoxate hydrochloride on voltage-dependent Ba2+ currents in human detrusor myocytes at different experimental temperatures

The inhibitory effects of flavoxate hydrochloride (piperidinoethyl-3-methylflavone-8-carboxylate; hereafter referred as flavoxate) on voltage-dependent nifedipine-sensitive inward Ba2+ currents (I Ba) in human detrusor myocytes were investigated at different temperatures using conventional whole-cell patch-clamp techniques. When the bath-solution temperature was increased from 22°C to 30°C, I Ba peak amplitude was enhanced by approximately twice at several test potentials. Neither the I Ba threshold nor the membrane potentials for the I Ba maximum peak amplitude was affected by the temperature change. The concentration-response curves of flavoxate at both 30°C (K i = 5.1 μM) and 37°C (K i = 4.6 μM) were slightly shifted to the left in comparison with that at 22°C (K i = 10.3 μM). Similar results were also obtained in the presence of nifedipine (K i = 14 nM at 22°C vs. K i = 2.5 nM at 30°C and K i = 2.1 nM at 37°C). Altering the bath-solution temperature from 22°C to 30°C shifted the steady-state inactivation curve of I Ba at −90 mV to the left. At 30°C, the steady-state inactivation curve of I Ba in the presence of flavoxate was also shifted to the left in comparison with that in the absence of flavoxate. Either 3-isobutyl-1-methylxanthine (IBMX) or theophylline, a phosphodiesterase inhibitor, caused little effects on I Ba, although cyclic nucleotides (dibutyryl cAMP and 8-Br-cGMP) inhibited I Ba. These results suggest that the inhibitory actions of flavoxate on I Ba in human detrusor myocytes were slightly changed at different experimental temperatures and that flavoxate directly blocked voltage-dependent L-type Ca2+ channels, not through the inhibition of phosphodiesterase activity pathway.