Ischemic and pharmacological preconditioning induces further delayed protection in transgenic mouse cardiac myocytes over-expressing adenosine A1 receptors (A1AR): role of A1AR, iNOS and K(ATP) channels.

In this study we examined the hypotheses that over-expression of the adenosine A(1) receptor (A(1)AR) in transgenic mouse cardiac myocytes (A(1)AR-tgm) induces cellular protection against subsequent sustained simulated ischemia (SI); that the cellular protection induced by over-expression of A(1)AR in A(1)AR-tgm is mediated through inducible nitric oxide synthase (iNOS) and K(ATP) channels. Sub-lethal simulated ischemia (SSI) and the A(1)AR agonists chloro- N(6)-cyclopentyl adenosine (CCPA) or (2 S)- N(6)-[2-endo-norbornyl]adenosine (S-ENBA) induce further, delayed cytoprotection, additional to the existing protection in A(1)AR-tgm. Cellular injury and cell viability was measured by the release of lactate dehydrogenase (LDH) or creatine kinase (CK) into the medium and the amount remaining in the cells. The cellular resistance acquired by cardiac myocytes due to the over-expression of A(1)AR was reflected by the reduced release of LDH (in units/liter) from 44.94+/-1.46 (wild-type mouse cardiac myocytes, wt) to 29.59+/-2.83 (A(1)AR-tgm, P<0.001). Conversely, LDH release from A(1)AR-tgm increased to 42.53+/-2.23 ( P<0.01) on exposure to 5-hydroxydecanoate (a mitochondrial K(ATP) channel blocker), to 45.93+/-2.90 ( P<0.01) on exposure to S-methylthiourea (an iNOS inhibitor) and to 56.04+/-3.00 ( P<0.01) on exposure to glibenclamide (a K(ATP) channel blocker). Treatment of A(1)AR-tgm is with SSI and the A(1)AR agonists chloro- N(6)-cyclopentyl adenosine (CCPA) or (2 S)- N(6)-[2-endo-norbornyl]adenosine (S-ENBA) decreased the release of LDH from 46.44+/-0.57 (A(1)AR-tgm) to 42.08+/-0.48 (A(1)AR-tgm plus SSI, P<0.05), 38.03+/-1.16 (A(1)AR-tgm plus CCPA, P<0.001) and 32.77+/-0.58 (A(1)AR-tgm plus S-ENBA, P<0.001). Our data suggest that the A(1)AR has a cytoprotective effect against subsequent sustained SI in A(1)AR-tgm and that the cellular protection induced by over-expression of A(1)AR in A(1)AR-tgm depends on iNOS and K(ATP) channels. Further, SSI and the A(1)AR agonists CCPA or S-ENBA induce further, delayed cytoprotection in A(1)AR-tgm.