Differential temperature-dependency of electrophysiological and inotropic actions of nifedipine, verapamil and cinnarizine in K+-depolarized ventricular myocardium

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 degrees C to 30 degrees 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 degrees C (Ki = 5.1 microM) and 37 degrees C (Ki = 4.6 microM) were slightly shifted to the left in comparison with that at 22 degrees C (Ki = 10.3 microM). Similar results were also obtained in the presence of nifedipine (Ki = 14 nM at 22 degrees C vs. Ki = 2.5 nM at 30 degrees C and Ki = 2.1 nM at 37 degrees C). Altering the bath-solution temperature from 22 degrees C to 30 degrees C shifted the steady-state inactivation curve of I Ba at -90 mV to the left. At 30 degrees 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.

Changes in cytosolic calcium in response to noxious heat and their relationship to vanilloid receptors in rat dorsal root ganglion neurons

Heat transduction mechanisms in primary nociceptive afferents have been suggested to involve a vanilloid receptor channel with high calcium permeability. To characterize the changes in free cytosolic calcium evoked by noxious heat stimuli (< or =51 degrees C, 10s), we performed microfluorometric measurements in acutely dissociated small dorsal root ganglion neurons (< or =32.5 microm) of adult rats using the dye FURA-2. Only neurons that responded with a reversible increase in intracellular calcium to high potassium were evaluated. Heat-induced calcium transients (exceeding mean + 3S.D. of the temperature dependence of the dye) were found in 66 of 105 neurons. These transients increased non-linearly with temperature. In contrast, heat-insensitive neurons showed a small linear increase of intracellular calcium throughout the range of 12-49 degrees C, similar to cardiac muscle cells. The vanilloid receptor agonist capsaicin induced calcium transients in 72 of 99 neurons. Capsaicin sensitivity and heat sensitivity were significantly associated (P<0.001, chi(2)-test), but 16 of 34 heat-insensitive cells responded to capsaicin and four of 49 heat-sensitive cells were capsaicin insensitive. The competitive vanilloid receptor antagonist capsazepine (10 microM) reversibly reduced the heat-induced calcium transients by 47+/-13%. In contrast, high potassium-induced calcium transients were not affected by pre-incubation with capsazepine. In calcium-free extracellular solution, the heat-induced rise in intracellular calcium was reduced by 76+/-5%. Heat-induced calcium transients were also reversibly reduced by 75+/-6% in sodium-free solution and by 62+/-7% with the L-type calcium channel blocker nifedipine (5 microM). These results indicate that noxious heat rapidly increases intracellular calcium in nociceptive primary sensory neurons. Heat-sensitive vanilloid receptors are involved in the induction of calcium transients, and calcium is also released from intracellular stores, but the main fraction of calcium passes through voltage-operated calcium channels.