Exaggerated coronary vasoreactivity to endothelin-1 in aged rats: Role of protein kinase C

Objective: The interaction between advanced age and increased susceptibility to ischemic insult is well documented. Age-related increases in coronary vascular resistance, in part due to impaired dilator responses, have been reported. Our aim was to determine the role of endothelin-1 (ET-1) on enhanced constrictor responses in aged coronary arteries (CAs) and whether protein kinase C (PKC) signaling mechanisms impact ET-1 responses. Methods: Vasoreactivity was assessed in CAs isolated from aged (24 months; n=16) and adult (4 months; n=21) male F344 rats following ET-1 (10^-10-10 ^-8) with and without specific ET_A/ET_B receptor antagonists (BQ-123, 1 μM; BQ-788, 30 nM) or the PKC inhibitor bisindolylmaleimide (Bis; 10^-6 M). Constrictor responses to KCl (80 mM) were also measured and voltage-gated Ca²⁺ channel (VGCC) determined in isolated coronary smooth muscle cells. Dilator responses to acetylcholine (ACH) and sodium nitroprusside (SNP) were assessed. Results: Passive diameter was greater (357±19 vs. 309±9; p<0.02) while spontaneous tone was similar in 24 months vs. 4 months. ET-1 resulted in greater constriction in 24 months vs. 4 months (79% vs. 67%; p<0.01). Group differences persisted following selective ET_B inhibition with BQ-788 (p<0.02), while BQ-123 abolished contractile responses to ET-1. Importantly, inhibition of ET-1 constriction by Bis occurred in 24 months but not 4 months (p<0.01). Constrictor responses to KCl and peak VGCC current density were similar in 24 months vs. 4 months (48% vs. 50%). No age-related differences were observed in ACH- or SNP-mediated dilation. Western blotting revealed increases in Ca²⁺-sensitive PKCα, -β_I, and -β_II levels with age, while eNOS and ET_A receptor protein levels were unchanged. Conclusion: Aberrant ET_A constrictor responses and directional changes in PKC are likely to contribute to coronary vascular pathology with advanced age.