Upregulation of cardiac sarcolemmal NCX current by insulin involves XIP interaction domain

VILLA-ABRILLE MC; SIDOR A; O'ROURKE B

Long Beach, EEUU.

Congreso; Annual Meeting of the Biophysical Society and International Biophysics Congress; 2008

: The cardiac sarcolemmal Na+/Ca2+ exchanger (NCX) plays an important role in Ca2+ extrusion and is thus essential for Ca2+ homeostasis. The large cytoplasmic domain (f-loop) participates in the regulation of NCX by various intracellular factors. Previously, residues 562-679 of the f-loop were show to be determinants of NCX inhibition by exchanger inhibitory peptide (XIP) (Maack et al, Circ. Res. 2005). Here we show that deletion of the same region eliminates the enhancement of NCX current by insulin, which has been reported to alter myocardial contractility. NCX current (INCX) was measured in adult guinea pig myocytes, either freshly isolated (FM) or cultured for up to 48 hrs (CM), and in myocytes after adenoviral expression of canine NCX1.1 with the 562-670 f-loop deletion (NCXD562-679). INCX was recorded by whole-cell patch clamp as the Ni2+-sensitive current at 37°C with intracellular Ca2+ buffered. Insulin (1 microM) increased INCX (at +80mV) in FM (n=9) and in CM (n=6) by 110% and 64%, respectively (p<0.05). The effect of insulin was decreased in myocytes expressing NCXD562-679 (+42%; n=7; N.S.) when XIP was not included in the pipette solution, and was further suppressed (+11%; n=9; N.S.) when XIP (100 microM) was included to block any remaining native guinea-pig INCX. The insulin effect on INCX was only partially inhibited by wortmannin (100nM), suggesting that the mechanism involves additional pathways besides phosphatidylinositol-3-kinase activation. The finding that the 562-679 f-loop domain is implicated in both XIP and receptor-mediated modulation of NCX highlight its important role in acute physiological or pathophysiological regulation of Ca2+ balance in the heart.