THE ROLE OF CAMP IN THE FREQUENCY-DEPENDENT CHANGES IN CONTRACTION OFGUINEA-PIG CARDIOMYOCYTES

Objectives: beta-Receptor desensitisation, low basal cAMP, and a negat ive force-frequency relationship are characteristic changes in human h eart failure. Isolated cardiomyocytes from noradrenaline-treated guine a pigs also show these features. We tested the hypothesis that low bas al cAMP underlies the loss of contractile response to increasing stimu lation frequency in this model, Methods: Isolated cardiomyocytes were obtained from noradrenaline-treated (NA) and sham-operated (SHAM) guin ea. Figs. They were stimulated from 0.1-2 Hz and contraction amplitude was monitored with a video edge-detection system. Results: NA cells h ad less positive amplitude-frequency responses (AFR) compared to SHAMs at 2 mM (P = 0.007, n = 17), or midrange Ca2+ concentrations (EC40-EC 60)(P < 0.001, n = 13). When the cAMP agonist, 8-CPT-cAMP (CPT, 10 mu M) or high Ca2+ (above EC75) was added to NA cells the AFR was normali sed to that of SHAM myocytes (NA vs. SHAM P = ns). In control experime nts the CAMP antagonists, Rp-cAMPS (Rpc) and Rp-8-CPT-cAMPS (Rp8, 100 mu M), blocked the positive inotropic effects of CPT at 0.5 Hz (contro l pD(2) = 4.36 +/- 0.06, Rp8 pD(2) = 3.68 +/- 0.08, P < 0.0001, n = 6 paired). Rpc (100 mu M) completely but reversibly blocked the effect o f maximal isoprenaline in control experiments (P < 0.0001). Neither an tagonist reduced the AFR compared to time-matched controls (P = ns, n = 6). Blockade of SERCA2a with thapsigargin resulted in a significant reduction in the AFR (ANOVA P < 0.0001). Conclusions: The results are consistent with sarcoplasmic reticulum (SR) function being a more impo rtant determinant of the amplitude-frequency relationship than tonic l evels of CAMP under basal conditions. Reversal of AFR depression by CP T may result from stimulation of SRCa2+ uptake. (C) 1998 Elsevier Scie nce B.V.