Lithium increases the alpha 1-adrenoceptor mediated inotropic effect in rat heart

Lithium opens store-operated channels in human platelets

Ca(2+) release from internal stores as a result of activation of phospholipase C or inhibition of the endoplasmic reticulum pump is accompanied by Ca(2+) influx from the extracellular space. Measurement of intracellular calcium concentration and fluorescence quenching in Fura2-loaded cells showed that platelets preincubated in lithium have significantly higher basal, but lower agonist-stimulated influx of Mn(2+) (acting as a surrogate of Ca(2+) influx), than platelets reloaded with calcium in a normal sodium medium. There is no difference in the basal entry of divalent ion in platelets preincubated in sodium, lithium, or N-methyl glucamine in the absence of calcium. In platelets preincubated in lithium there is a higher basal Mn(2+) entry without further increase upon store depletion by thapsigargin. In contrast, a significant increase in the divalent ion influx was found in sodium or N-methyl glucamine attributable to the opening of channels sensitive to store depletion. In the absence of extracellular calcium, the empty store opens channels and Li(+) did not have additional effect on channels that are already open. The refilling of the stores with Ca(2+) suppresses Mn(2+) entry after sodium or NMG preincubation, but not after lithium preincubation. We propose that lithium induces a calcium influx throughout store-operated channels. This hypothesis may explain the lack of additivity, in cell preincubated in lithium, of basal entry and thapsigargin-triggered entry of calcium.

Myocardial contractile response and IP3, cAMP and cGMP interrelationships

An experimental study in the perfused working normal and pressure overloaded rat heart. A mini review based on a doctoral thesis.

Prazosin reduces myocardial ischemia/reperfusion-induced Ca2+ overloading in rat heart by inhibiting phosphoinositide signaling

The aim of this study was to establish whether or not alpha 1-adrenergic receptors are implicated in triggering phosphoinositide hydrolysis and intracellular Ca2+ accumulation during myocardial ischemia and reperfusion. In isolated perfused rat hearts, the selective alpha 1-receptor antagonist prazosin abolished the increase in radioactivity incorporation into cellular inositol phosphates induced by 30 min ischemia followed by 30 min reperfusion, and selectively blocked the degradation of phosphoinositides; only minor changes in the ischemia/reperfusion-induced loss of other classes of phospholipids were seen. In addition, a prazosin-induced decrease of ischemia/reperfusion Ca2+ overloading was documented in real-time recordings of epicardial cytosolic free Ca2+ in fura 2-loaded hearts. An inhibition of early ischemic Ca2+ rise was observed, as well as a lower peak of cytosolic free Ca2+ and a more rapid reversal to normal values during reperfusion. Moreover, alpha 1-adrenergic blockade resulted in a significant improvement in the recovery of myocardial function during reperfusion: an increased left ventricular developed pressure and maximum rate of rise of systolic pressure paralleled the decrease in time-averaged cytosolic Ca2+ and the increase in amplitude of Ca2+ transients, respectively. It is concluded that myocardial Ca2+ overloading during ischemia and reperfusion may be triggered by alpha 1-adrenergic receptor-induced polyphosphoinositide hydrolysis.