Positive inotropic responses to calcitonin gene-related peptide (CGRP) were evaluated in atria isolated from in vivo rat models of hyper-sensory-motor innervation (following neonatal guanethidine treatment) and hypo-sensory-motor innervation (following neonatal capsaicin treatment), to explore the hypothesis that functional responsiveness of atrial myocardium to CGRP may correlate with tissue levels of the sensory-motor neurotransmitters. Comparative of inotropic responses to CGRP following in vitro treatment of atria with guanethidine was also performed. 2. Following long-term guanethidine treatment, positive inotropic responses to CGRP were significantly attenuated, while supersensitivity to the sympathetic transmitter noradrenaline was shown. Maximal inotropic responses to CGRP (30 nM) were 214.0 +/- 28.1 (n = 8) and 146.8 +/- 21.7 mg (n = 8; P < 0.01) increase of the basal contractile tension in control and treated preparations, respectively. The pD2 values for noradrenaline were 6.71 +/- 0.12 (n = 8) and 7.26 +/- 0.13 (n = 6; P < 0.01) in control and treated atria, respectively. Acute application of guanethidine in vitro did not modify the positive inotropism by CGRP or the beta-adrenoceptor agonist isoprenaline. 3. Sensory-motor hypoinnervation following chronic treatment with capsaicin did not affect the inotropic responses to CGRP. Neither guanethidine nor capsaicin treatment affected the contractile apparatus of myocytes, as demonstrated by similar basal contractile tension as well as calcium-evoked inotropic responses in control and treated preparations. 4. In summary, increased sensory-motor innervation, following long-term sympathectomy with guanethidine, resulted in attenuation of the inotropic responses of the rat atrium to CGRP, while no changes in the inotropic responses were seen following sensory-motor denervation with capsaicin. Down-regulation of CGRP receptors or altered post-receptor signalling may be involved in the reduced responsiveness to CGRP.
