Increased cAMP as a positive inotropic factor for mammalian skeletal muscle in vitro

To test the hypothesis that an increased cAMP concentration improves skeletal muscle force development, we stimulated mouse soleus and extensor digitorum longus (EDL) in the presence of isoproterenol (1 x 10(-5) mol.L-1), a beta-adrenergic agonist, or N6,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate (dcAMP) (1 x 10(-3) mol.L-1), a membrane-permeable cAMP analogue. Drugs used in the challenges were dissolved in Krebs-Henseleit bicarbonate buffer (Krebs) at 27 degrees C and gassed with 95% O2 - 5% CO2. Stimulation at 50 impulses.s-1 for 0.5 s produced an isometric tetanic contraction. Over 25 min of contractions at 0.6 contractions.min-1, developed force increased significantly with the addition of isoproterenol (soleus, 2.5% +/- 1.1%; EDL, 13.8% +/- 2.0%) or dcAMP (soleus, 2.3% +/- 0.5%; EDL, 10.9% +/- 1.9%) as compared with vehicle controls (cont) with Krebs added (soleus, 0.0% +/- 0.2%; EDL, -2.5% +/- 0.7%). To investigate the role of Ca2+ availability, we amplified or attenuated sarcolemmal L-type Ca2+ channels with Bay K 8644 (Bay K) (5.6 x 10(-6) mol.L-1) or diltiazem hydrochloride (dilt) (10(-4) mol.L-1), respectively. Ca2+ release from the sarcoplasmic reticulum was increased with caffeine (2 x 10(-3) mol.L-1) or decreased with dantrolene sodium (dant) (4.2 x 10(-7) mol.L-1). With Ca2+availability modified, dcAMP addition in soleus significantly increased force development above control (cont, 2.3% +/- 0.4%; Bay K, 4.0% +/- 1.0%; dilt, 52.3% +/- 3.6%; caffeine, 2.3% +/- 0.7%; dant, 6.0% +/- 2.0%; dilt + dant, 55.0% +/- 23.0%). In EDL, the addition of dcAMP also increased force development above control (cont, 13.7% +/- 1.9%; Bay K, 17.0% +/- 4.0%; dilt, 170.0% +/- 40.0%; caffeine, 23.0% +/- 4.0%; dant, 72.0% +/- 10.0%; dilt + dant, 54.0% +/- 14.0%). Thus, a positive inotropic effect of cAMP existed in both fast- and slow-twitch mammalian skeletal muscle with both normal and altered Ca2+ flux into the sarcoplasm.