Overexpression of junctate induces cardiac hypertrophy and arrhythmia via altered calcium handling.

Junctate-1 is a newly identified integral endoplasmic/sarcoplasmic reticulum Ca2+ binding protein. However, its functional role in the heart is unknown. In the present study, the consequences of constitutively overexpressed junctate in cardiomyocytes were investigated using transgenic (TG) mice overexpressing junctate-1. TG mice (8 weeks old) showed cardiac remodeling such as marked bi-atrial enlargement with intra-atrial thrombus and biventricular hypertrophy. The TG mice also showed bradycardia with atrial fibrillation, reduced amplitude and elongated decay time of Ca2+ transients, increased L-type Ca2+ current and prolonged action potential durations. Time-course study (2-8 weeks) showed an initially reduced SR function due to down-regulation of SERCA2 and calsequestrin followed by sarcolemmal protein expression and cardiac hypertrophy at later age. These sequential changes could well be correlated with the physiological changes. Adrenergic agonist treatment and subsequent biochemical study showed that junctate-1 TG mice (8 weeks old) were under local PKA signaling that could cause increased L-type Ca2+ current and reduced SR function. Junctate-1 in the heart is closely linked to the homeostasis of E-C coupling proteins and a sustained increase of junctate-1 expression leads to a severe cardiac remodeling and arrhythmias.

Norepinephrine activates store-operated Ca2+ entry coupled to large-conductance Ca2+-activated K+ channels in rat pinealocytes.

Norepinephrine (NE) is one of the major neurotransmitters that determine melatonin production in the pineal gland. Although a substantial amount of Ca(2+) influx is triggered by NE, the Ca(2+) entry pathway and its physiological relevance have not been elucidated adequately. Herein we report that the Ca(2+) influx triggered by NE significantly regulates the protein level of serotonin N-acetyltransferase, or arylalkylamine N-acetyltransferase (AANAT), a critical enzyme in melatonin production, and is responsible for maintaining the Ca(2+) response after repetitive stimulation. Ca(2+) entry evoked by NE was dependent on PLC activation. NE evoked a substantial amount of Ca(2+) entry even after cells were treated with 1-oleoyl-2-acetyl-sn-glycerol (OAG), an analog of diacylglycerol. To the contrary, further OAG treatment after cells had been exposed to OAG did not evoke additional Ca(2+) entry. Moreover, NE failed to induce further Ca(2+) entry after the development of Ca(2+) entry induced by thapsigargin (Tg), suggesting that the pathway of Ca(2+) entry induced by NE might be identical to that of Tg. Interestingly, Ca(2+) entry evoked by NE or Tg induced membrane hyperpolarization that was reversed by iberiotoxin (IBTX), a specific inhibitor of large-conductance Ca(2+)-activated K(+) (BK) channels. Moreover, IBTX-sensitive BK current was observed during application of NE, suggesting that activation of the BK channels was responsible for the hyperpolarization. Furthermore, the activation of BK channels triggered by NE contributed to regulation of the protein level of AANAT. Collectively, these results suggest that NE triggers Ca(2+) entry coupled to BK channels and that NE-induced Ca(2+) entry is important in the regulation of AANAT.

Excitatory effect of M1 muscarinic acetylcholine receptor on automaticity of mouse heart.

We have investigated the effects of relatively high concentration of carbachol (CCh), an agonist of muscarinic acetylcholine receptor (mAChR), on cardiac automaticity in mouse heart. Action potentials from automatically beating right atria of mice were measured with conventional microelectrodes. When atria were treated with 100 microM CCh, atrial beating was immediately arrested and diastolic membrane potential (DMP) was depolarized. After exposure of the atria to CCh for approximately 4 min, action potentials were regenerated. The regenerated action potentials had lower frequency and shorter duration when compared with the control. When atria were pre-exposed to pirenzepine (1 microM), an M1 mAChR antagonist, there was complete inhibition of CCh-induced depolarization of DMP and regeneration of action potentials. Pre-exposure to AFDX-116 (11 ({2-[(diethylamino)-methyl]-1 -piperidyl}acetyl)-5,11 -dihydro-6H-pyridol[2,3-b][1,4] benzodiazepine-6-one base, 1 microM), an M2 mAChR antagonist, failed to block CCh-induced arrest of the beating. However, prolonged exposure to CCh elicited gradual depolarization of DMP and slight acceleration in beating rate. Our data indicate that high concentration of CCh depolarizes membrane potential and recovers right atrial automaticity via M1 mAChR, providing functional evidence for the role of M1 mAChR in the atrial myocytes.