AbstractAdenylyl cyclase type 9 (AC9) is found tightly associated with the scaffolding protein Yotiao and the I_Ks ion channel in heart. But apart from potential I_Ks regulation, physiological roles for AC9 are unknown. We show that loss of AC9 in mice reduces less than 3% of total AC activity in heart but eliminates Yotiao-associated AC activity. AC9^−/− mice exhibit no structural abnormalities but show a significant bradycardia, consistent with AC9 expression in sinoatrial node. Global changes in PKA phosphorylation patterns are not altered in AC9^−/− heart, however, basal phosphorylation of heat shock protein 20 (Hsp20) is significantly decreased. Hsp20 binds AC9 in a Yotiao-independent manner and deletion of AC9 decreases Hsp20-associated AC activity in heart. In addition, expression of catalytically inactive AC9 in neonatal cardiomyocytes decreases isoproterenol-stimulated Hsp20 phosphorylation, consistent with an AC9-Hsp20 complex. Phosphorylation of Hsp20 occurs largely in ventricles and is vital for the cardioprotective effects of Hsp20. Decreased Hsp20 phosphorylation suggests a potential baseline ventricular defect for AC9^−/−. Doppler echocardiography of AC9^−/− displays a decrease in the early ventricular filling velocity and ventricular filling ratio (E/A), indicative of grade 1 diastolic dysfunction and emphasizing the importance of local cAMP production in the context of macromolecular complexes.