Effect of phosphorylation on L-type calcium current in ventricular myocytes dialysed with proteolytic enzymes

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

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.

Effects of the class III antiarrhythmic agent dofetilide (UK-68,798) on L-type calcium current from rabbit ventricular myocytes

The methanesulphonanilide agent dofetilide (UK-68,798) exerts Class III antiarrhythmic effects by inhibiting the cardiac rapid delayed rectifier potassium current (I(Kr)) encoded by HERG. The aim of the present study was to determine whether dofetilide also exhibits Class IV (L-type calcium-channel blocking) effects. L-type calcium current (I(Ca,L)) was measured from rabbit isolated ventricular myocytes, using the whole-cell patch-clamp technique under selective recording conditions. Positive control experiments demonstrated inhibition of I(Ca,L) elicited by pulses to + 10 mV by both nifedipine and externally applied Ni2+ ions. Three concentrations of dofetilide were tested: 100 nM, 1 microM and 10 microM. I(Ca,L) magnitude was not significantly reduced by any of the concentrations tested (P > 0.05; n = minimum of seven cells per drug concentration). The inactivation time-course of I(Ca,L) was also unaffected by 10 microM dofetilide. Heterologously expressed HERG current (I(HERG)) recorded from Chinese Hamster Ovary cells was extensively inhibited by 100 nm and 1 microM dofetilide, with inhibition at 1 microM not significantly different from 100% (P > 0.1). It is concluded that dofetilide produced no I(Ca,L) blocking effects at concentrations up to and exceeding that required for maximal I(HERG) inhibition. The findings support the notion that dofetilide is a highly selective Class III antiarrhythmic agent, devoid of Class IV antiarrhythmic activity.

Reprogramming of gene expression in cultured cardiomyocytes and in explanted hearts by the myosin ATPase inhibitor butanedione monoxime

BACKGROUND

Butanedione monoxime (BDM) is a reversible myosin ATPase inhibitor. Its use in transplantation medicine may be of benefit in the preservation of hearts. As little is known about its ability to prevent stress and metabolic deregulation, we wanted to investigate the genomic response in cultured cardiomyocytes and explanted, preserved hearts at the transcriptional level.

METHODS

We thus investigated the gene expression of the transcription factors GATA-4, Nkx2.5, MEF-2c, and Oct-1 and of the downstream target genes atrial and brain natriuretic peptide, alpha- and beta-myosin heavy chain, alpha-cardiac actin, and alpha-skeletal actin. Additionally, lactate dehydrogenase and creatine kinase enzyme activities were measured as markers for membrane integrity and metabolic deregulation of cardiomyocytes.

RESULTS

In untreated cardiomyocyte cultures, expression of GATA-4 and Nkx2.5 was increased 7- and 4-fold, 72 hr after isolation, but the gene expression of MEF-2c and Oct-1 was reduced to 10% and 70%, at day 3 in culture. We show atrial natriuretic peptide and brain natriuretic peptide gene expression to be maximal 24 and 72 hr after isolation, the level being 3- and 2-fold, when compared with freshly isolated cells. The gene expression of alpha- and beta-myosin heavy chain was reduced to approximately 30% at day 3 in culture and similar observations were made for alpha-cardiac and alpha-skeletal actin, which declined to approximately 20% and 10% of control values, 72 hr after isolation. BDM prevented at the transcriptional level enhanced expression of markers for stress and metabolic deregulation, and the activities of lactate dehydrogenase and creatine kinase were highly significantly reduced. Similar results were obtained when explanted hearts were stored in BDM-containing organ preservation solution.

CONCLUSIONS

Preservation of metabolic function in donor organs is of critical importance in transplantation medicine, and we show gene markers for stress and metabolic deregulation in cultures of cardiomyocytes and explanted hearts to be significantly reduced by BDM. Reprogramming of gene expression of nuclear transcription factors and downstream target genes may prolong the acceptable storage time between explantation and transplantation.