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Glucose-dependent insulinotropic polypeptide formerly called gastrin inhibitory peptide (GIP), a peptide composed of 42 amino acids, is formed in duodenal and jejunal cells. GIP acts via GIP receptors (GIPR). GIPR can stimulate adenylyl cyclases (AC) and increase intracellular cyclic adenosine-3´,5´-monophosphate (cAMP) levels. The physiological role of GIPR in the human heart is not fully understood. Thence, force of contraction (FOC) was studied in isolated electrically driven (1 Hz) human right atrial preparations from patients undergoing bypass surgery due to severe coronary heart disease. We noted that in paced human atrium, GIP increased FOC. This effect was reduced by a GIPR-antagonist (ProGIP). In the presence of 0.1 µM cilostamide, a phosphodiesterase (PDE) 3 inhibitor, the positive inotropic effects (PIE) of GIP were more potent and efficient to raise FOC. Up to 100 nM GIP failed to heighten the spontaneous beating rate in mouse right atrial preparations, but increased FOC in electrically driven left atrial mouse preparations but only in the presence of a PDE 4 inhibitor (100 nM rolipram). We conclude that the human atrium and the mouse atrium contain functional GIPR with respect to FOC. Show less

Retatrutide (LY3437943) was developed as a drug to treat type 2 diabetes and obesity. Retatrutide, a not endogenously occurring peptide, stimulated the glucagon receptor (GCGR), the glucose-dependent insulinotropic polypeptide (GIP) receptor (GIPR), and the glucagon-like peptide-1 receptor (GLP-1R) in cell cultures; increased the activity of adenylyl cyclases (AC); and thus augmented the 3',5' cyclic adenosine monophosphate (cAMP) levels. We tested the hypothesis that retatrutide increased force of contraction (FOC) in human right atrial preparations (HAP) from adult patients. HAP were obtained during open heart surgery from patients who suffered from severe coronary heart disease. We noted that cumulatively applied retatrutide starting at 10 nM (up to 100 nM the highest concentration tested) elevated FOC in HAP in a concentration- and time-dependent manner. In the additional presence of the phosphodiesterase III inhibitor cilostamide (1 µM), retatrutide was more potent and more effective to increase FOC in HAP. Under these conditions, retatrutide shortened the time of muscle relaxation in HAP. These positive inotropic effects of glucagon were diminished by a GLP1-R antagonist, by a GIPR antagonist, and by a CGCR antagonist but not by propranolol, an antagonist at β-adrenoceptors. The effects of retatrutide on FOC were also reduced by 100 nM ryanodine, an inhibitor of the ryanodine receptor in the sarcoplasmic reticulum, by 1 µM carbachol, a M-cholinoceptor agonist, and by 1 µM (-)-N Show less

When retatrutide stimulates the glucagon receptor (GCGR), the glucose-dependent insulinotropic polypeptide (GIP) receptor (GIPR), and the glucagon-like peptide-1 receptor (GLP-1R), then 3',5'cyclic adenosine monophosphate (cAMP) is increased. We tested the hypothesis that retatrutide like the β-adrenoceptor agonist isoprenaline raises force of contraction (FOC) in isolated electrically driven (1 Hz) left atrial preparations (LA) and exerts positive chronotropic effects (PCE) in isolated spontaneously beating right atrial preparations (RA) from adult CD1 mice. While 100 nM isoprenaline increased FOC, retatrutide (100 nM) failed to increase FOC in LA. In isolated mouse right atrial preparations (RA), retatrutide exerted PCE that were potentiated by 100 nM rolipram but that were antagonized by adomeglivant, a GCGR antagonist. The PCE of retatrutide but not the PCE of isoprenaline were attenuated by H89, an inhibitor of the cAMP-dependent protein kinase (PKA). The PCE of retatrutide were not weakened by the β-adrenoceptor antagonist propranolol (1 µM) but were blocked by 1 µM carbachol, an agonist at M Show less

Tirzepatide is an approved drug that is used to treat type 2 diabetes. Tirzepatide is a peptide comprised of 39 amino acids and activates glucose-dependent insulinotropic polypeptide receptors (GIPR) and glucagon-like peptide-1 receptors (GLP-1R). Via GIPR and GLP-1R, tirzepatide stimulated in cell culture adenylyl cyclases (AC) and thereby elevated the cellular content of 3':5' cyclic adenosine monophosphate (cAMP). We tested the hypothesis that tirzepatide augmented the force of contraction (FOC) in isolated electrically driven (1 Hz) human right atrial preparations (HAP) obtained during open heart surgery from adult patients. Cumulatively applied tirzepatide, starting at nanomolar concentrations, raised FOC in a concentration-dependent manner and a time-dependent manner (p < 0.05). The positive inotropic effects (PIE) of tirzepatide were attenuated by about a quarter by a GIPR antagonist (100 nM, Pro3-GIP) and by about three quarters by a GLP-1R antagonist (100 nM, exendin9-39) in HAP. Tirzepatide (1 µM) was less effective than 1 µM isoprenaline in raising FOC in HAP. The inhibitor of the cAMP-dependent protein kinase called H89 reversed the PIE of tirzepatide. We suggest that tirzepatide probably acts via stimulation of GIPR and GLP-1R to exert a PIE in HAP. Show less