Beta1-adrenergic receptors stimulate cardiac contractility and CaMKII activation in vivo and enhance cardiac dysfunction following myocardial infarction

Abstract

The beta-adrenergic receptor (betaAR) signaling system is one of the most powerful regulators of cardiac function and a key regulator of Ca(2+) homeostasis. We investigated the role of betaAR stimulation in augmenting cardiac function and its role in the activation of Ca(2+)/calmodulin-dependent kinase II (CaMKII) using various betaAR knockouts (KO) including beta(1)ARKO, beta(2)ARKO, and beta(1)/beta(2)AR double-KO (DKO) mice. We employed a murine model of left anterior descending coronary artery ligation to examine the differential contributions of specific betaAR subtypes in the activation of CaMKII in vivo in failing myocardium. Cardiac inotropy, chronotropy, and CaMKII activity following short-term isoproterenol stimulation were significantly attenuated in beta(1)ARKO and DKO compared with either the beta(2)ARKO or wild-type (WT) mice, indicating that beta(1)ARs are required for catecholamine-induced increases in contractility and CaMKII activity. Eight weeks after myocardial infarction (MI), beta(1)ARKO and DKO mice showed a significant attenuation in fractional shortening compared with either the beta(2)ARKO or WT mice. CaMKII activity after MI was significantly increased only in the beta(2)ARKO and WT hearts and not in the beta(1)ARKO and DKO hearts. The border zone of the infarct in the beta(2)ARKO and WT hearts demonstrated significantly increased apoptosis by TUNEL staining compared with the beta(1)ARKO and DKO hearts. Taken together, these data show that cardiac function and CaMKII activity are mediated almost exclusively by the beta(1)AR. Moreover, it appears that beta(1)AR signaling is detrimental to cardiac function following MI, possibly through activation of CaMKII.

Fig. 1.

Representative case of basic hemodynamic parameters (A–D) and pressure-volume (P-V) loop with end-systolic elastance (E_es) in mice. Basal heart rate (HR) and peak rise in pressure (dP/dt_max) are markedly increased following isoproterenol (Iso) infusion in wild-type (WT; A) and β₂-adrenergic receptor knockout (β₂ARKO; C) mice. There is no increase in β₁ARKO (B) or double knockout (DKO; D) mice. The P-V relationships display no change after isoproterenol in mice lacking β₁ARs (i.e., β₁ARKO and DKO; G and H), whereas contractility is increased in the WT (E) and β₂ARKO mice (F).

Fig. 2.

Hemodynamic changes (%) of baseline value after isoproterenol stimulation in mice (n = 8–9 for each group). In WT and β₂ARKO mice, heart rate, dP/dt_max, maximal power, and E_es are significantly increased compared with basal levels, with no significant increase seen in β₁AR or DKO mice (A–D). *P < 0.005 WT or β₂AR vs. β₁AR or DKO, #P < 0.05 WT or β₂AR vs. β₁AR or DKO.

Fig. 3.

Isoproterenol-stimulated Ca²⁺/calmodulin-dependent kinase II (CaMKII; A) and phospholamban (PLB; B) activity in mice (n = 4 for each group). A: representative immunoblot showing accumulation of phospho-CaMKII after treatment with 10 μg·g⁻¹·min⁻¹ isoproterenol infusion in WT and β₂ARKO mice. Summary graph represents a significant increase of phospho-CaMKII in WT and β₂ARKO mice. B: representative immunoblot showing accumulation of phospho-PLB after treatment with 10 μg·g⁻¹·min⁻¹ isoproterenol infusion in WT and β₂ARKO mice. Summary graph represents a significant increase of Ser-16 (left) and Thr-17 (right) in WT and β₂ARKO mice. *P < 0.005, treatment with isoproterenol vs. without isoproterenol for each group.

Fig. 4.

β₁ARs promote detrimental cardiac remodeling in WT and β₂AR KO mice post-MI. A: histological sections of infarcted hearts are shown at 8 wk post-MI. B: representative M-mode echocardiograms of WT and KO mice pre-MI (Pre) and at 4 wk (4W) and 8 wk (8W) post-MI. C–E: serial echocardiography in conscious WT and βARKO mice after MI. C: % left ventricular end-diastolic dimension (LVEDD) change. D: % left ventricular end-systolic dimension (LVESD) change. E: % fractional shortening (FS) change. *P < 0.005 β₁AR (n = 13) or DKO (n = 15) vs. β₂AR (n = 16) or WT (n = 18) at 8 wk. †P < 0.005 β₁AR vs. WT, β₂AR, or DKO at 4 wk.

Fig. 5.

βAR density and CaMKII activity following MI in βAR knockout mice (n = 5 for each group). A: only β₁ARs are downregulated 8 wk post-MI, whereas receptor densities of β₂ARs remain unchanged. *P < 0.05 sham vs. MI within group. B: CaMKII activity post-MI. C: sham hearts. The presence of β₁AR stimulates Ca²⁺-dependent CaMKII activity after MI. *P < 0.05 Ca²⁺ dependent vs. Ca²⁺ independent within group.

Fig. 6.

β₁ARs promote apoptotic cell death after MI (n = 6 for each group). A: border zone. B: remote zone. Increased terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL)-positive nuclei were identified in WT and β₂AR KO mice after 8 wk MI in border zone. No difference in rate of apoptosis at remote zone. *P < 0.05 WT or β₂AR vs. β₁AR or DKO.