Temperature-sensitive intracellular Mg block of L-type Ca channels in cardiac myocytes

Yamaoka, Kaoru, Tsunetsugu Yuki, Kayoko Kawase, Makoto Munemori, and Issei Seyama. Temperature-sensitive intracellular Mg2 block of L-type Ca2 channels in cardiac myocytes. Am J Physiol Heart Circ Physiol 282: H1092–H1101, 2002. First published November 1, 2001; 10.1152/ajpheart.00585.2001.—We examined the concentration-dependent blocking effects of intracellular Mg2 on Ltype Ca2 channels in cardiac myocytes using the whole cell patch-clamp technique. The increase of L-type Ca2 channel current (ICa) (due to relief of Mg2 block) occurred in two temporal phases. The rapid phase (runup) transiently appeared early ( 5 min) in dialysis of the low-Mg2 solution; the slow phase began later in dialysis ( 10 min). Runup was not blocked by intracellular GTP (GTPi). The late phase of the ICa increase (late ICa) was suppressed by GTPi (0.4 mM) and was observed in myocytes of the guinea pig or frog at higher (32 or 24°C, respectively) rather than lower temperatures (24 or 17.5°C, respectively). At pMg 6.0, raising the temperature from 24 to 32°C evoked late ICa with a Q10 of 14.5. Restoring the temperature to 24°C decreased ICa with a Q10 of only 2.4. The marked difference in the Q10 values indicated that late ICa (pMg 5–6) is an irreversible phenomenon. Phosphorylation suppressed the intracellular [Mg2 ] dependency of late ICa. This effect of phosphorylation together with the inhibitory action of GTPi on Mg2 -dependent blocking of ICa are common properties of mammalian and amphibian cardiomyocytes.