Calcitonin Gene-Related Peptide Stimulates a Positive Contractile Response in Rat Ventricular Cardiomyocytes

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Abstract
Calcitonin gene-related peptide (CGRP) elicits marked positive inotropic and chronotropic actions in the atria of several mammals. The second-messenger substance cyclic AMP and activation of L-type calcium channels have been implicated in these actions, but CGRP failed consistently to stimulate a contractile response in ventricular tissue obtained from various mammals. We assessed the actions of CGRP using isolated ventricular cardiomyocytes obtained from adult rats. Maximum changes in cell length (dL) of isolated cardiomyocytes during electrically stimulated (0.5 Hz) contractions were determined with adenosine deaminase (2.5 U/ml). In these conditions, CGRP produced a potent concentration-dependent positive contractile response that became maximal 4 min after initial stimulation. CGRP increased amplitude of cellular contractions maximally at a 1-nM concentration to a value 21.4% greater than that obtained without peptide. The EC50 value for the response was 31 pM. At concentrations greater than 1 nM, amplitude of the cellular contractile response decreased rapidly. The CGRP2-selective agonist, [cys ACM2,7] CGRP, increased the amplitude of cellular contractions maximally at 500 nM to a value 19.8% greater than that obtained without peptide. EC50 for this response was 6 nM. Salmon calcitonin (< or = 100 nM) did not elicit a significant contractile response. The fragment, CGRP8-37, a selective antagonist at the CGRP1 receptor subtype, while devoid of agonist activity, was a potent competitive antagonist of the positive contractile action of CGRP (pA2 value = 7.95). CGRP, present at maximally effective concentration (1 nM), when combined with isoprenaline ISO 100 pM-1 microM, elicited a greater increase in contractile amplitude than that elicited by ISO 100 pM-1 microM without CGRP. CGRP 1 nM combined with low concentrations of extracellular calcium ion < or = 4 mM produced a greater increase in contractile amplitude than that elicited by calcium ion < or = 4 mM without CGRP, but this additive effect was abolished in the presence of higher concentrations of extracellular calcium ion (> 4 mM). The cyclic AMP antagonist, Rp-cyclic AMPS (< or = 200 microM), did not inhibit the contractile response to CGRP 1 nM, but inhibited the contractile responses to ISO 100 nM and secretin 20 nM significantly and in a concentration-dependent manner. Diltiazem < or = 1 microM, a selective antagonist of L-type calcium channels, also failed to inhibit the contractile response to CGRP 1 nM but inhibited the contractile responses to ISO 100 nM and secretin 20 nM significantly and in a concentration-dependent manner.(ABSTRACT TRUNCATED AT 400 WORDS)

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