In some forms of cardiac hypertrophy and failure, the gain of Ca(2+)-induced Ca(2+) release [CICR; i.e., the amount of Ca(2+) released from the sarcoplasmic reticulum normalized to Ca(2+) influx through L-type Ca(2+) channels (LTCCs)] decreases despite the normal whole cell LTCC current density, ryanodine receptor number, and sarcoplasmic reticulum Ca(2+) content. This decrease in CICR gain has been proposed to arise from a change in dyad architecture or derangement of the t-tubular (TT) structure. However, the activity of surface sarcolemmal LTCCs has been reported to increase despite the unaltered whole cell LTCC current density in failing human ventricular myocytes, indicating that the "decreased CICR gain" may reflect a decrease in the TT LTCC current density in heart failure. Thus, we analyzed LTCC currents of failing ventricular myocytes of mice chronically treated with isoproterenol (Iso). Although Iso-treated mice exhibited intact t-tubules and normal LTCC subunit expression, acute occlusion of t-tubules of isolated ventricular myocytes with osmotic shock (detubulation) revealed that the TT LTCC current density was halved in Iso-treated versus control myocytes. Pharmacological analysis indicated that kinases other than PKA or Ca(2+)/calmodulin-dependent protein kinase II insufficiently activated, whereas protein phosphatase 1/2A excessively suppressed, TT LTCCs in Iso-treated versus control myocytes. These results indicate that excessive β-adrenergic stimulation causes the decrease in TT LTCC current density by altering the regulation of TT LTCCs by protein kinases and phosphatases in heart failure. This phenomenon might underlie the decreased CICR gain in heart failure.

Whether nicorandil is effective at preventing ventricular tachyarrhythmia (VT) during acute myocardial ischaemia is still controversial. We examined effects of nicorandil on the induction of VT during acute myocardial ischaemia. Optical action potentials were recorded from the entire transmural wall of arterially perfused canine left ventricular wedges. Ischaemia was produced by arterial occlusion for 20 min. During endocardial pacing, nicorandil shortened mean action potential duration (APD) in the transmural wall before ischaemia and further shortened it during ischaemia without increasing dispersion of APD. HMR1098, a selective blocker of sarcolemmal ATP-sensitive K + channels, inhibited the shortening of APD by nicorandil before and during ischaemia. Ischaemia decreased transmural conduction velocity (CV). Nicorandil partially restored CV to a similar extent in the absence and presence of HMR1098. In contrast, HMR1098 did not suppress the ischaemic conduction slowing in the absence of nicorandil. Nicorandil suppressed the increased dispersion of local CV during ischaemia. Isochrone maps on the initiation of VT showed that reentry in the transmural surface resulted from the excitation of the epicardial region of transmural surface. Nicorandil significantly increased the size of non-excited area in the epicardial region of the transmural wall, thereby significantly reducing the incidence of VT induced during ischaemia. HMR1098 inhibited this effect of nicorandil. These results suggest that nicorandil prevents VT during acute global ischaemia primarily by augmenting the inactivation of epicardial muscle through the activation of sarcolemmal K ATP channels.

It is well known that parathyroid hormone (PTH) possesses an anabolic effect on bone. However, the mechanisms are not fully elucidated. So far, it is unclear whether or not PTH could stimulate the expression of bone morphogenetic protein-2 (BMP-2), a strong mediator for bone formation. Growing evidence suggests that BMP-2 expression is regulated by the mevalonate pathway and Rho-associated protein kinase (ROK) activity. This study was performed to examine if PTH affects BMP-2 expression and to clarify its involvement of the mevalonate pathway. Osteoblastic MC3T3-E1 cells were treated with human PTH-(1-34) to determine BMP-2 mRNA expression levels by real-time PCR and to measure the ROK activity by the kinase assay. Incubation with 10 (-9)-10 (-8) M of hPTH-(1-34) for 6 h induced significant upregulation of BMP-2 mRNA levels in MC3T3-E1 cells. Short-term treatment of hPTH-(1-34) suppressed Rho kinase activity and mevalonate kinase mRNA levels. PTH-induced BMP-2 mRNA upregulation was selectively reversed by geranylgeranyl pyrophosphate (GGPP) pretreatment, but not by mevalonate pretreatment. These findings suggest that BMP-2 mRNA expression was upregulated by PTH in MC3T3-E1 cells mediated by mevalonate pathway suppression followed by ROK inhibition. We have now demonstrated for the first time that PTH stimulated BMP-2 mRNA expression via the mevalonate pathway and ROK in osteoblastic MC3T3-E1 cells.

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