Inhibition of the calcium channel by intracellular protons in single ventricular myocytes of the guinea‐pig.

Abstract

SUMMARY1. The inhibitory effects of intracellular protons (Hi) on the L-type Ca2l channel activity were investigated in single ventricular myocytes of guinea-pigs by using the patch-clamp method in the open-cell-attached patch configuration, where 'run down' of the channel was partially prevented.2. Hi reduced the unitary Ba2+ current of the Ca21 channel by 10-20 % without changing the maximum slope conductance.3. Hi did not alter the number of channels in patches containing one or two channels.4. Hi markedly reduced the mean current normalized by the unitary current, which gave the open-state probability multiplied by the number of channels in the patch. The dose-response curve between Ht and the open-state probability indicated half-maximum inhibition at pHi 6-6 and an apparent Hill coefficient of 1.5. H+ shifted both the steady-state activation and inactivation curves in a negative direction by 10-15 mV, and the effects were reversible.6. H+ did not affect the fast open-closed kinetics represented by the C-C-O scheme, apart from increasing the slow time constant of the closed time.7. Hi increased the percentage of blank sweeps and reduced that of non-blank sweeps resulting in a decreased probability of channel opening.8. Photo-oxidation with Rose Bengal abolished the reducing effect of Hi on the open-state probability (P.) in two out of ten experiments, suggesting the possible involvement of histidine residues in the Ht effect. 9. The above results indicate that Ht inhibits the Ba2+ current mainly by affecting the slow gating mechanism of the channel.

The effects of extracellular and intracellular pH on intracellular Ca2+ regulation in guinea‐pig detrusor smooth muscle

“…However, the lack of effect of intracellular H¤ on the Ca¥ transient evoked by high-K¤ suggested that the L_type channel was insensitive to pH changes in this compartment, and this was corroborated by direct measurement of ICa during variation of pHé. This insensitivity of the L-type Ca¥ channel to pHé changes is in contrast to the situation in other muscles where intracellular acidosis is effective in reducing the magnitude of ICa (Kaibara & Kameyama, 1988). The attenuation of the carbachol-and caffeine-induced Ca¥ transients by extracellular acidosis is less straightforward.…”

Effects of intracellular pH on calcium currents and intracellular calcium ions in the smooth muscle of rabbit portal vein

“…DISCUSSION NH4C1 markedly potentiated and propionate inhibited Ca2`channel currents in smooth muscle cells of the rabbit portal vein and these effects are likely to be mediated by intracellular alkalinization and acidification, respectively. Similar effects of pH1 have been reported for cardiac muscle (Irisawa & Sato, 1986;Kaibara & Kameyama, 1988). In guinea-pig ventricular muscle cells, studied by the patch-clamp method in the open-cellattached configuration with 50 mm Ba2`and Bay K 8644 in the patch pipettes, inhibition of L-type Ca2`channels by decreasing pHi is shown to be mainly due to a decreased probability of channel opening (Kaibara & Kameyama, 1988).…”

“…In the pulmonary artery (Krampetz & Rhoades, 1991) and the rat mesenteric artery (Matthews, Graves & Poston, 1992), the transient contraction elicited on removal of NH Cl is inhibited by Ca21 channel blockers, such as nifedipine or veralpalmil, suggesting that Ca21' channels may be activated by intracellular acidification. It has been reported, however, in cardiac muscle cells that L-type Ca'2 channels are inhibited by internal protons (Irisawa & Sato, 1986;Kaibara & Kameyama, 1988). Therefore, in the present experiments, the effects of pH, on L-type Ca2' channels in smooth muscle cells isolated from the rabbit portal vein were studied using NH C1 and propionate to modify pH Changes in intracellular Ca2' concentration ([Ca 2j]i) were also estimated using ftira-2 and mechanical responses were recorded simultaneously.…”

Regulation of acetylcholine release by intracellular acidification of developing motoneurons in Xenopus cell cultures

“…In hippocampal CA1 neurons, Ca¥ channels were reversibly depressed by moderate extracellular acidosis (pH 6·0-6·9) and enhanced slightly by alkaline exposure (pH 8·0) (Tombaugh & Somjen, 1996). On the other hand, alkaline intracellular pH enhanced and acidic intracellular pH inhibited vascular and cardiac L-type Ca¥ currents by affecting the channel availability andÏor channel open probability (Kaibara & Kameyama, 1988;Iino, Hayashi, Saito, Tokuno & Tomita, 1994). However, T-type Ca¥ channel activity was decreased by pHï acidification, but not affected by internal protons (Tytgat et al 1990).…”