GPR55 exerts differential effects on cardiac adrenoceptor subtypes in mice

SK Walsh, CL Wainwright. Robert Gordon University, Aberdeen, UK

In heart failure (HF), cardiac sympathetic activity is increased leading to the chronic stimulation of cardiac β1-adrenoceptors (β1-ARs), which results in the subsequent loss of these receptors and reduced AR mediated inotropy [1]. Recent data from our group has demonstrated that mice with a gene deletion for G protein coupled receptor 55 (GPR55) are characterised by cardiac decompensation (i.e. reduced contractile reserve) following dobutamine administration [2] and the present study was carried out to identify the AR subtype(s) affected by GPR55 gene deletion and determine their role in the cardiac decompensation observed in these mice. Cardiac function was assessed via pressure volume loop (PVL) analysis in male (3 month old; 28-29g) wild-type (WT) and homozygous GPR55 knockout (GPR55-/-) mice. Mice were anaesthetised with a mixture of ketamine & xylazine (120mg/kg & 16mg/kg i.p., respectively) and a 1.4-Fr pressure conductance catheter (SPR-839; Millar Instruments) inserted into the left ventricle via the right carotid artery to record cardiac function. Post stabilisation, baseline cardiac function was recorded in all mice and then the following pharmacological agents administered to both WT and GPR55-/- mice and the change from baseline function calculated: Group 1 (n=9 for each genotype) – dobutamine (10μg kg-1; α1/β1/β2-AR agonist); Group 2 (n=7-8) – procaterol (0.02-2μg kg-1; β2-AR agonist); Group 3 (n=9 for both) – A-61603 (0.2-20μg kg-1; α1-AR agonist) & Group 4 (n=9 for both) - dobutamine (1-10μg kg-1) in the presence of both prazosin (α1-AR antagonist) and ICI 118,551 (β2-AR antagonist; both 1mg kg-1 i.p.) to unmask the β1-AR response. PVL analysis of basal cardiac function revealed that GPR55-/- mice were characterised by increased SBP, DBP, MABP, HR and CO (all P<0.05) compared to WT mice. In contrast, GPR55-/- mice were characterised by reduced contractile reserve in response to dobutamine alone. β2-adrenoceptor activation had minimal effects on cardiac function that did not differ between genotypes. Administration of dobutamine, in the presence of both prazosin and ICI 118,551, to GPR55-/- mice revealed significantly attenuated β1-AR mediated pressor (end systolic pressure (ESP); 4±1 vs. 14±1mmHg), lusitropic (dP/dt min; -82±174 vs. -1013±175mmHg/μL), and inotropic (dP/dt max (1354±128 vs. 2047±145mmHg/μL) & ejection fraction (2±1.2 vs. 12±4%)) responses compared to WT mice (all P<0.05). Furthermore, β1-AR induced increases in SV and CO were attenuated in GPR55-/- mice, while β1-AR induced changes in chronotropy were unaffected by genotype. In contrast, the α1-AR agonist, A-61603, induced significantly increased pressor (ESP; 77±6 vs. 60±2mmHg), lusitropic (dP/dt min; -3013±312 vs. 205±100mmHg/μL), and inotropic (dP/dt max; 5759±469 vs. 3450±256mmHg/μL) responses and significantly increased arterial elastance (18±2 vs. 12.5±0.6mmHg/μL) in GPR55-/- mice compared to WT mice (all P<0.05). Finally, A-61603 induced significant reductions in SV, EF and CO in WT mice were attenuated in GPR55-/- mice. While A-61603 induced changes in cardiac function were almost entirely abrogated by prazosin in WT mice, this antagonist only partially reversed the effects of A-61603 in GPR55-/- mice (all P<0.05). Our findings demonstrate that GPR55 influences adrenoceptor function in the heart, exerting opposing effects on adrenoceptor subtypes. Therefore GPR55 may play a possible role in the altered adrenoceptor signalling characteristic of heart failure.

1. Mohl et al. (2011) Cardiovasc Res 91: 310-319.

2. Walsh et al. (2014) PLoS One 9(10):e108999

This work was funded by an IPF Pump Priming Award from the British Pharmacological Society (2013).