Bimodal regulation of Na+-Ca2+ exchanger by beta -adrenergic signaling pathway in shark ventricular myocytes

Abstract

In shark heart, the Na ؉ -Ca 2؉ exchanger serves as a major pathway for both Ca 2؉ influx and efflux, as there is only rudimentary sarcoplasmic reticulum in these hearts. The modulation of the exchanger by a ␤-adrenergic agonist in whole-cell clamped ventricular myocytes was compared with that of the Na ؉ -Ca 2؉ exchanger blocker KB-R7943. Application of 5 M isoproterenol and 10 M KB-R7943 suppressed both the inward and the outward Na ؉ -Ca 2؉ exchanger current (I Na؊Ca ). The isoproterenol effect was mimicked by 10 M forskolin. Isoproterenol and forskolin shifted the reversal potential (Erev) of INa؊Ca by approximately ؊23 mV and ؊30 mV, respectively. An equivalent suppression of outward I Na؊Ca by KB-R7943 to that by isoproterenol produced a significantly smaller shift in Erev of about ؊4 mV. The ratio of inward to outward exchanger currents was also significantly larger in isoproterenolthan in control-and KB-R7943-treated myocytes. Our data suggest that the larger ratio of inward to outward exchanger currents as well as the larger shift in Erev with isoproterenol results from the enhanced efficacy of Ca 2؉ efflux via the exchanger. The protein kinase A-mediated bimodal regulation of the exchanger in parallel with phosphorylation of the Ca 2؉ channel and enhancement of its current may have evolved to satisfy the evolutionary needs for accelerated contraction and relaxation in hearts of animals with vestigial sarcoplasmic Ca 2؉ release stores. The ␤-adrenergic agonists are known to potentiate the force of cardiac contraction by enhancing Ca 2ϩ current (I Ca ) and Ca 2ϩ release from the internal calcium store secondary to cAMP-dependent phosphorylation of the Ca 2ϩ channel (1-3). Relaxant effect of catecholamines (4), on the other hand, appears to be mediated by phosphorylation of phospholamban and troponin C resulting in stimulation of the Ca 2ϩ pump (5, 6) and decreased myofilament Ca 2ϩ sensitivity (7-9). Similar to the mammalian myocardium, the shark heart also exhibits some of the features associated with ␤-agonist response, namely enhancement of I Ca (10) and twitch force, and suppression of maintained contracture tension (11). In light of previous reports showing little ultrastructural and functional evidence for Ca 2ϩ release stores in shark heart (12, 13), the ␤-adrenergic-induced suppression of KCl-induced contractures (11) may be somewhat surprising. In shark heart the Na ϩ -Ca 2ϩ exchanger appears to serve as a major pathway of both Ca 2ϩ influx and efflux (13). Here we report on a cellular mechanism by which the ␤-adrenergic agonists suppress the influx and efflux mode of the Na ϩ -Ca 2ϩ exchanger, but enhance the efficacy of Ca 2ϩ efflux (I inward ͞I outward ) via the exchanger. To gain insight into the mechanism by which isoproterenol regulates the Na ϩ -Ca 2ϩ exchanger, we compared the effects of this hormone with that of the exchanger blocker . Our experiments reveal that in shark myocytes this bimodal regulation of the exchanger may decrease the Ca 2ϩ load of the myocytes during experimentally ...

Effect of β-adrenergic stimulation on the relationship between membrane potential, intracellular [Ca2+] and sarcoplasmic reticulum Ca2+ uptake in rainbow trout atrial myocytes

“…It would also be in agreement with the absence, in both the trout and mammalian NCX protein (Nicoll et al, 1990;Xue et al, 1999), of the putative PKA-dependent phosporylation site described in frog (Fan et al, 1996). It is, however, contrary to results from the frog heart, where a putative PKA-dependent phosporylation site on the NCX has been described (Fan et al, 1996) and a PKA-dependent inhibition of NCX activity has been reported in both frog and shark cardiac myocytes (Fan et al, 1996;Woo and Morad, 2001). The present results also show that the importance of SR Ca 2+ uptake relative to sarcolemmal Ca 2+ extrusion by the NCX is unchanged with β-adrenergic stimulation in trout atrial myocytes.…”

Molecular determinants of cAMP-mediated regulation of the Na+-Ca2+ exchanger expressed in human cell lines

“…Therefore, the suppression of inward I Na–Ca by 8‐Br‐cAMP, and the resulting shift of the reversal potential, in part reflects a decrease in the amount of Ca 2+ brought into the cell during the conditioning pulse (cf. Woo & Morad, 2001). The results in Fig.…”

Catecholamines, cardiac natriuretic peptides and chromogranin A: evolution and physiopathology of a ‘whip-brake’ system of the endocrine heart

“…Moreover, CA modify the vasoactivity of the coronary system [welldeveloped in elasmobranchs ], thus adjusting coronary performance to myocardium requirements (Axelsson, 1995). An electrophysiological study in sharks (Woo and Morad, 2001) (Woo and Morad, 2001).…”