PUFA and aging modulate cardiac mitochondrial membrane lipid composition and Ca²⁺ activation of PDH

Aberrations in cell Ca²⁺ homeostasis have been known to parallel both changes in membrane lipid composition and aging. Previous work has shown that the lowered efficiency of work performance, which occurs in isolated hearts from rats fed a diet rich in n-6 polyunsaturated fatty acids (PUFA), relative to those fed n-3 PUFA, could be raised by mitochondrial (Mito) Ca²⁺ transport inhibition. We tested whether, after Ca²⁺-dependent stress, the Ca²⁺-dependent activation of pyruvate dehydrogenase (PDH(A)/PDH(Total)) and Mito Ca²⁺ cycling could be manipulated by varying the ratio of n-3 to n-6 PUFA in Mito membranes in young (6 mo) and aged (24 mo) isolated rat hearts treated to n-3 or n-6 PUFA-rich diet. Inotropic stimulation by 1 μM norepinephrine (NE) of 24-mo n-6 PUFA-rich hearts elevated total Mito Ca²⁺ content 38% more than in 6-mo hearts (P < 0.05). However, both the NE- induced rise in Mito Ca²⁺ and the difference in response between 6- and 24- mo hearts were partially abolished by n-3 PUFA treatment. NE increased the fractional activation of PDH by 44% above control levels in the 6-mo group compared with 49% in the 24-mo group after n-6 PUFA diet. However, NE stimulation of PDHA was attenuated by n-3 PUFA diet, attaining values only 29 and 23% above control levels in 6- and 24-mo mitochondria, respectively (P < 0.05). Global ischemia and reperfusion (I/R) in n-6 PUFA hearts gave rise to higher levels of total Mito Ca²⁺ concentration (P < 0.0001) and PDHA (P < 0.0001) compared with n-3 PUFA. Ruthenium red (3.4 μM) abolished the effects of I/R in all groups. With aging, heart Mito membrane phosphatidylcholine was increased after n-6 PUFA-rich diet (by -15%, P < 0.05), whereas cardiolipin and n-3 PUFA content were diminished by 31% (P < 0.05) and 73% (P < 0.05), respectively. These effects were prevented by n-3 PUFA-rich diet. The present study, by directly manipulating the cardiac Mito membrane n-3-to-n-6 PUFA ratio, shows that the activation of Ca²⁺-dependent PDH can be augmented when the n-3-to-n-6 PUFA ratio is low (n-6 PUFA-rich diet; 24-mo hearts) or attenuated when this ratio is relatively high (n-3 PUFA-rich diet). We propose that one of the consequences of dietary-induced manipulation of membrane phospholipids and PUFAs may be the altered flux of Ca²⁺ across the Mito membrane and thus altered intramitochondrial Ca²⁺-dependent processes.