title: 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 (GTP(i)). The late phase of the I(Ca) increase (late I(Ca)) was suppressed by GTP(i) (0.4 mM) and was observed in myocytes of the guinea pig or frog at higher (32 or 24 degrees C, respectively) rather than lower temperatures (24 or 17.5 degrees C, respectively). At pMg = 6.0, raising the temperature from 24 to 32 degrees C evoked late I(Ca) with a Q10 of 14.5. Restoring the temperature to 24 degrees 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 GTP(i) on Mg2+-dependent blocking of I(Ca) are common properties of mammalian and amphibian cardiomyocytes.
authors.affiliation: Department of Physiology, School of Medicine, Hiroshima University, Minami-Ku, Hiroshima 734-8551, Japan. kyamaok@hiroshima-u.ac.jp
authors.forename: Kaoru
authors.initials: K
authors.lastname: Yamaoka
authors.forename: Tsunetsugu
authors.initials: T
authors.lastname: Yuki
authors.forename: Kayoko
authors.initials: K
authors.lastname: Kawase
authors.forename: Makoto
authors.initials: M
authors.lastname: Munemori
authors.forename: Issei
authors.initials: I
authors.lastname: Seyama
mesh_terms: D000818:Animals; D020746:Calcium Channels, L-Type / Q000187:drug effects / Q000502:physiology*; D006160:Guanosine Triphosphate / Q000494:pharmacology; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects / Q000502:physiology*; D006352:Heart Ventricles; D066298:In Vitro Techniques; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology / Q000502:physiology*; D008297:Male; D011898:Ranidae; D013696:Temperature; D013997:Time Factors
chemical_list: D020746:Calcium Channels, L-Type; D006160:Guanosine Triphosphate; D008274:Magnesium