Kazuya Horike

Atrial natriuretic peptide stimulates Na+-dependent Ca2+ efflux from freshly isolated adult rat cardiomyocytes

In the present study, we examined the effect of atrial natriuretic peptide (ANP) on Ca2+ efflux from freshly isolated adult rat cardiomyocytes. Rat ANP(1–28) stimulated the efflux of 45Ca2+ from the cells in a concentration‐dependent manner (10−8 M to 10−6 M). The 45Ca2+ efflux was not affected by removal of extracellular Ca2+, but was dependent on the presence of extracellular Na+. In addition, rat ANP(1–28) caused 22Na+ influx into the cells. The 45Ca2+ efflux was also stimulated by C‐type natriuretic peptide‐22 (CNP‐22), but not by rat brain natriuretic peptide‐45 (BNP‐45). It was also observed that both rat ANP(1–28) and CNP‐22 stimulated guanosine 3′,5′‐cyclic monophosphate production within the cells. These results indicate that ANP stimulates Na+‐dependent 45Ca2+ efflux from freshly isolated adult rat cardiomyocytes, probably through Na+/Ca2+ exchange, and that the stimulatory effect of ANP on Ca2+ efflux may be mediated via the natriuretic peptide receptor which has been shown to couple to guanylate cyclase. Since it is reported that Na+/Ca2+ exchange is important in calcium homeostasis within cells, ANP may play a role in the extrusion of intracellular Ca2+ from isolated adult rat cardiomyocytes.

Effect of Amiloride on Ischaemia and Reperfusion Injury in Isolated, Perfused Rat Hearts

The effect of amiloride, a potent inhibitor of Na+/H+ exchange, on ischaemic reperfused rat hearts was studied in order to investigate whether Na+/H+ exchange or Na+/Ca2+ exchange is involved in ischaemia-reperfusion injury, When hearts were pre-ischaemically loaded with 100 microM amiloride, recovery of left ventricular developed pressure was significantly better than in control hearts, whereas recovery of heart rate at 30-min reperfusion was unaffected. Amiloride pretreatment also decreased creatine phosphokinase activity in the coronary effluent and completely abolished occurrence of ventricular arrhythmias during reperfusion. It also inhibited intracellular Na+ accumulation early in reperfusion (within 5 min), whereas in the late stage (from 5 to 30 min), Ca2+ overload was inhibited. The findings suggest that Na+/H+ exchange participates mainly in the early stage of reperfusion injury and the Na+/Ca2+ exchange system, secondary to Na+/H+ exchange, in the late stage. The reduction in post-ischaemic cardiac dysfunction induced by amiloride pretreatment may be attributable to inhibition of the resultant Ca2+ accumulation during reperfusion.

Neuropeptide Y as a stimulator of Na+-dependent Ca2+ efflux from freshly isolated adult rat cardiomyocytes

Several physiological stimuli cause a rise in intracellular Ca2+ concentration ([Ca2+]i) in cardiomyocytes. This increased [Ca2+]i must be restored to physiological resting level to ensure response to further stimuli. In the present study, we examined the effect of neuropeptide Y (NPY), which is secreted from certain adrenergic or non-adrenergic neurons, on Ca2+ efflux from freshly isolated, quiescent adult rat cardiomyocytes. The isolated cardiomyocytes were preloaded with 45CaCl2 for 1 h. Then, the fractional release of 45Ca2+ from the cells was measured. NPY stimulated the efflux of 45Ca2+ from isolated adult rat cardiomyocytes in a concentration-dependent manner (10–8 M to 10–6 M). NPY (10–6 M)-induced Ca2+ efflux was 2.0 ± 0.16% of the total cellular content. The 45Ca2+ efflux from the cells was also stimulated by Y1 receptor agonist, [Leu31, Pro34]NPY, but not by Y2 receptor agonist, NPY13–36. The effect of NPY was inhibited by a peptide NPY inhibitor, NPY18–36 and a non-peptide NPY inhibitor, benextramine to a similar extent. From these results, it is conceivable that the effect of NPY on Ca2+ efflux from cardiomyocytes is mediated through Y1 receptors. It was also observed that NPY caused a rise in [Ca2+]i to almost 150 nM. NPY-stimulated 45Ca2+ efflux was not affected by removal of extracellular Ca2+, but was dependent on the presence of extracellular Na+. Moreover, NPY caused a 22Na+ influx into the cells of about 1.6-fold over the basal value which was inhibited by amiloride and 5-(N,N-dimethyl)-amiloride, known Na+/Ca2+ exchange inhibitors. In addition, isoproterenol also caused 45Ca2+ efflux from the cells and which was enhanced by the addition of NPY. These results suggest that NPY stimulates extracellular Na+-dependent 45Ca2+ efflux from freshly isolated adult rat cardiomyocytes, probably through its stimulatory effect on plasma membrane Y1 receptors with which NPY may couple during Na+/Ca2+ exchange.

A Recurrent Case of Infectious Endocarditis after Mitral Valve Repair.

僧帽弁形成術後に人工腱索に発症したと考えられる感染性心内膜炎の再発例を経験した.症例は36歳,男性.1998年12月感染性心内膜炎,僧帽弁前尖の腱索断裂による4度の僧帽弁逆流と診断され約2ヵ月の内科的治療ののちに人工腱索を使用した僧帽弁形成術を行い良好に経過していた.2000年2月感冒様症状を契機として僧帽弁前尖に疣贅の形成を伴った感染性心内膜炎を再発し,疣贅の塞栓による脳梗塞を併発するなど急性に増悪を示した.感染は化学療法にて鎮静化したが,弁破壊による僧帽弁逆流が出現し再手術を施行した.初回手術時人工腱索を縫着していた前尖の部位に強い炎症所見と穿孔を認めた.炎症が広範囲であったため人工弁置換術を行った.感染性心内膜炎では,起炎菌の同定や抗生剤の有効性,脳合併症の有無,弁や支持組織の破壊進行の程度などが治療方針の決定および予後の改善に大きく影響する.また本症例では基礎疾患に免疫機能不全の存在も疑われた.