Temperature-sensitive intracellular Mg2+ block of L-type Ca2+ channels in cardiac myocytes

Abstract

We examined the concentration-dependent blocking effects of intracellular Mg2+ on L-type Ca2+ channels in cardiac myocytes using the whole cell patch-clamp technique. The increase of L-type Ca2+ channel current ( I Ca) (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 I Ca increase (late I Ca) 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 I Ca with a Q10 of 14.5. Restoring the temperature to 24°C decreased I Ca with a Q10 of only 2.4. The marked difference in the Q10 values indicated that late I Ca (pMg = 5–6) is an irreversible phenomenon. Phosphorylation suppressed the intracellular [Mg2+] dependency of late I Ca. This effect of phosphorylation together with the inhibitory action of GTPi on Mg2+-dependent blocking of I Ca are common properties of mammalian and amphibian cardiomyocytes.