title: The role of cAMP in the frequency-dependent changes in contraction of guinea-pig cardiomyocytes.
abstract: OBJECTIVE
  beta-Receptor desensitisation, low basal cAMP, and a negative force-frequency relationship are characteristic changes in human heart failure. Isolated cardiomyocytes from noradrenaline-treated guinea pigs also show these features. We tested the hypothesis that low basal cAMP underlies the loss of contractile response to increasing stimulation frequency in this model.
  
  
  METHODS
  Isolated cardiomyocytes were obtained from noradrenaline-treated (NA) and sham-operated (SHAM) guinea pigs. They were stimulated from 0.1-2 Hz and contraction amplitude was monitored with a video edge-detection system.
  
  
  RESULTS
  NA cells had less positive amplitude-frequency responses (AFR) compared to SHAMs at 2 mM (P = 0.002, n = 17), or midrange Ca2+ concentrations (EC40-EC60) (P < 0.001, n = 13). When the cAMP agonist, 8-CPT-cAMP (CPT, 10 microM) or high Ca2+ (above EC75) was added to NA cells the AFR was normalised to that of SHAM myocytes (NA vs. SHAM P = ns). In control experiments the cAMP antagonists, Rp-cAMPS (Rpc) and Rp-8-CPT-cAMPS (Rp8, 100 microM), blocked the positive inotropic effects of CPT at 0.5 Hz (control pD2 = 4.36 +/- 0.06, Rp8 pD2 = 3.68 +/- 0.08, P < 0.0001), n = 6 paired). Rpc (100 microM) completely but reversibly blocked the effect of maximal isoprenaline in control experiments (P < 0.0001). Neither antagonist 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 important determinant of the amplitude-frequency relationship than tonic levels of cAMP under basal conditions. Reversal of AFR depression by CPT may result from stimulation of SR Ca2+ uptake.
authors.affiliation: Imperial College School of Medicine, National Heart and Lung Institute, London, UK.
authors.forename: A R
authors.initials: AR
authors.lastname: Money-Kyrle
authors.forename: C H
authors.initials: CH
authors.lastname: Davies
authors.forename: H K
authors.initials: HK
authors.lastname: Ranu
authors.forename: P
authors.initials: P
authors.lastname: O'Gara
authors.forename: N S
authors.initials: NS
authors.lastname: Kent
authors.forename: P A
authors.initials: PA
authors.lastname: Poole-Wilson
authors.forename: S E
authors.initials: SE
authors.lastname: Harding
mesh_terms: D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D048429:Cell Size / Q000187:drug effects; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000031:analogs & derivatives / Q000037:antagonists & inhibitors / Q000494:pharmacology / Q000502:physiology*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006168:Guinea Pigs; D006333:Heart Failure / Q000378:metabolism*; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects*; D009206:Myocardium / Q000378:metabolism*; D009638:Norepinephrine / Q000494:pharmacology; D019284:Thapsigargin / Q000494:pharmacology; D013873:Thionucleotides / Q000494:pharmacology
chemical_list: D000318:Adrenergic beta-Agonists; D004791:Enzyme Inhibitors; D013873:Thionucleotides; C016957:adenosine-3',5'-cyclic phosphorothioate; C015929:8-((4-chlorophenyl)thio)cyclic-3',5'-AMP; D019284:Thapsigargin; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D000252:Calcium-Transporting ATPases; D007545:Isoproterenol; D002118:Calcium; D009638:Norepinephrine