Submitted on November 16, 2005
Accepted on December 20, 2005
Inositol phospholipids localized to caveolae in rat heart are regulated by 
1-adrenergic receptors and by ischemia/reperfusion
Alfred A Lanzafame1, Lynne Turnbull2, Fatemeh Amiramahdi1, Jane F Arthur3, Huy Huynh1, and Elizabeth A Woodcock1*
1 Cellular Biochemistry, Baker Heart Research Institute, Melbourne, Victoria, Australia
2 Microbiology, Monash University, Melbourne, Victoria, Australia
3 Biochemistry, Monash University, Melbourne, Victoria, Australia
* To whom correspondence should be addressed. E-mail: liz.woodcock{at}baker.edu.au.
Post-ischemic reperfusion of rat or mouse hearts causes generation of inositol(1,4,5)trisphosphate (Ins(1,4,5)P3) and the initiation of arrhythmias. In the current study we investigated the possibility that the enhanced Ins(1,4,5)P3 generation in post-ischemic reperfusion was associated with an increased availability of the precursor lipid phosphatidylinositol(4,5)bisphosphate (PIP2) for 
1-adrenergic receptor activated phospholipase C (PLC). Isolated-perfused rat hearts were labeled with [3H]inositol and subjected to ischemia/reperfusion or stimulation with norepinephrine under normoxic conditions. Caveolar fractions were prepared by buoyant density sucrose gradient centrifugation. [3H]PIP2 was concentrated in caveolae, along with Gaq and PLC
1b. Caveolae contained only 27.3 ± 6.9 % (mean ± SEM, n=6) of the total1-adrenergic receptor complement of the heart. These did not migrate to PIP2-containing fractions with nnorepinephrine stimulation under normoxic conditions, even though caveolar PIP2 was depleted. In contrast, [3H]PIP2 in caveolae increased during 2 min reperfusion, independently of norepinephrine release and thus of 1-adrenergic receptor activation. The increased PIP2 in the caveolar fractions where signaling proteins are concentrated may be critical for the heightened generation of Ins(1,4,5)P3 in early reperfusion.
