Downstream defects in beta-adrenergic signaling and relation to myocyte contractility after cardioplegic arrest

Transient left ventricular dysfunction can occur after hypothermic, hyperkalemic cardioplegic arrest and is associated with decreased beta-adrenergic receptor responsiveness. Occupancy of the beta-adrenergic receptor activates adenylate cyclase, which phosphorylates the L-type Ca2+ channel-enhancing myocyte contractility. The goal of this study was to identify potential mechanisms that contribute to the defects in the beta-adrenergic receptor signaling cascade after cardioplegic arrest.

Isolated left ventricular porcine myocytes were assigned to one of two treatment groups: (1) cardioplegic arrest (24 mEq/L K+, 4 degrees C x 2 hours, then 5 minutes in 37 degrees C cell media; n = 130) or (2) normothermic control (cell media, 37 degrees C x 2 hours; n = 222). Myocyte contractility was assessed at baseline and after either beta-adrenergic receptor occupancy (25 nmol/L isoproterenol [INN: isoprenaline]), activation of adenylate cyclase (0.5 mumol forskolin), or direct activation of the L-type Ca(2+)-channel (10 nmol/L or 100 nmol/L (-)BayK 8644).

Myocyte velocity of shortening (micron/sec) was increased with beta-adrenergic receptor occupancy or direct adenylate cyclase stimulation compared with baseline in the normothermic group (187.3 +/- 6.9, 181.7 +/- 10.2, and 73.9 +/- 2.9, respectively; p < 0.0001) and after cardioplegic arrest (128.6 +/- 8.9, 124.3 +/- 9.4, and 46.1 +/- 2.6, respectively; p < 0.0001). However, the response after cardioplegic arrest was significantly reduced compared with normothermic values under all conditions (p = 0.012). Direct activation of the L-type Ca(2+)-channel, which eliminates beta-adrenergic receptor-dependent events, increased myocyte contractility in the normothermic group (161.90 +/- 12.0, p < 0.0001) and after cardioplegic arrest (92.78 +/- 6.8, p < 0.0001), but the positive inotropic response appeared reduced compared with normothermic control values (p = 0.003).

These findings suggest that contributory mechanisms for the reduced beta-adrenergic receptor-mediated response after hypothermic, hyperkalemic cardioplegic arrest lie downstream from these specific components of the transduction pathway and likely include defects in Ca2+ homeostasis, myofilament Ca2+ sensitivity, or both.