CALCITONIN-GENE-RELATED PEPTIDE STIMULATES A POSITIVE CONTRACTILE RESPONSE IN RAT VENTRICULAR CARDIOMYOCYTES

Calcitonin gene-related peptide (CGRP) elicits marked positive inotrop ic and chronotropic actions in the atria of several mammals. The secon d-messenger substance cyclic AMP and activation of L-type calcium chan nels have been implicated in these actions, but CGRP failed consistent ly to stimulate a contractile response in ventricular tissue obtained from various mammals. We assessed the actions of CGRP using isolated v entricular cardiomyocytes obtained from adult rats. Maximum changes in cell length (dL) of isolated cardiomyocytes during electrically stimu lated (0.5 Hz) contractions were determined with adenosine deaminase ( 2.5 U/ml). In these conditions, CGRP produced a potent concentration-d ependent positive contractile response that became maximal 4 min after initial stimulation. CGRP increased amplitude of cellular contraction s maximally at a 1-nM concentration to a value 21.4% greater than that obtained without peptide. The EC(50) value for the response was 31 pM . At concentrations greater than 1 nM, amplitude of the cellular contr actile response decreased rapidly. The CGRP(2)-selective agonist, [cys ACM(2,7)] CGRP, increased the amplitude of cellular contractions maxi mally at 500 nM to a value 19.8% greater than that obtained without pe ptide. EC(50) for this response was 6 nM. Salmon calcitonin (less than or equal to 100 nM) did not elicit a significant contractile response . The fragment, CGRP(8-37), a selective antagonist at the CGRP(1) rece ptor subtype, while devoid of agonist activity, was a potent competiti ve antagonist of the positive contractile action of CGRP (pA(2) value = 7.95). CGRP, present at maximally effective concentration (1 nM), wh en combined with isoprenaline ISO 100 pM-1 mu M, elicited a greater in crease in contractile amplitude than that elicited by ISO 100 pM-1 mu M without CGRP. CGRP 1 nM combined with low concentrations of extracel lular calcium ion less than or equal to 4 mM produced a greater increa se in contractile amplitude than that elicited by calcium ion less tha n or equal to 4 mM without CGRP, but this additive effect was abolishe d in the presence of higher concentrations of extracellular calcium io n (>4 mM). The cyclic AMP antagonist, Rp-cyclic AMPS (less than or equ al to 200 CGM), 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 l ess than or equal to 1 mu M, 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. With phosph odiesterase (PDE) inhibitor, isobutyl methylxanthine (IBMX 1 mM) and a denosine deaminase 2.5 U/ml added, CGRP less than or equal to 10 mu M and salmon calcitonin less than or equal to 10 mu M failed to stimulat e production of cyclic AMP in suspensions of viable cardiomyocytes, wh ereas secretin, included as a positive control, produced maximal stimu lation four times greater than that obtained without peptide. CGRP les s than or equal to 10 mu M did not stimulate production of cyclic GMP in suspensions of viable cardiomyoctyes, whereas sodium nitroprusside (SNP), used as a positive control, stimulated production maximally to a value 2.75 times greater than that obtained without agonist. These d ata indicate that CGRP exerts a potent positive contractile response i n rat ventricular cardiomyocytes at concentrations compatible with tho se circulating in rat plasma. The response is mediated by receptors of the CGRP, subtype but is not coupled to a cyclic AMP- or cyclic GMP-d ependent intracellular pathway and does not involve recruitment of L-t ype calcium channels.