Abstract
Sympathetic nervous system dysfunction is prevalent in diabetes. It is characterised by increased sympathetic nerve activity and elevated circulating catecholamine levels and altered β-adrenoceptor (β-AR) responsiveness. β-AR are critical in regulating chronotropy (heart rate, HR), inotropy (myocardial contraction), lusitropy (myocardial relaxation) and systemic vasodilation, key components of blood pressure regulation. However, the contribution of specific β1- and β2-AR subtypes in diabetes is unclear. Therefore, this thesis aimed to determine the cardiovascular responsiveness of specific β1- and β2-AR stimulation in type II diabetes.
To investigate the β-AR responsiveness in diabetes, telemetric recording of arterial blood pressure (ABP) measured the mean ABP (MAP) and HR responses induced by pharmacological β-AR stimulation following specific β1- AR blockade in conscious leptin receptor-deficient 20-week-old male Zucker diabetic fatty (ZDF) rats. Diabetic alterations in β-AR stimulated chronotropy and vascular responses were associated with increased myocardial β1-AR responsiveness and an absence of β1-AR evoked coronary vasodilation in vivo. Systemic vascular β-AR responsiveness was decreased in diabetic rats.
To determine whether these alterations in β-AR chronotropic responsiveness observed in vivo were intrinsic or extrinsic to the heart and to investigate the β-AR subtype contributions to inotropy, HR and contractile responses were recorded during pharmacological β-AR stimulation following specific β1-AR or β2-AR blockade in isolated heart preparations. The lower HR in diabetes was preserved ex vivo and remained lower throughout β-AR stimulation. β-AR chronotropic responsiveness was unchanged in isolated diabetic hearts but inotropic and lusitropic β-AR responsiveness, as indicated by the maximum rate of contraction (+dP/dtmax) and maximum rate of relaxation (-dP/dtmax), were reduced in diabetic rats. Similar by β1-AR stimulated +dP/dtmax and -dP/dtmax were impaired in diabetic hearts and atrial-paced rhythms of 5 Hz (300 beats min-1 ) did not normalise inotropy or lusitropy. In contrast specific β2-AR stimulation did not stimulate chronotropy or inotropy, though dual stimulation of β2-AR with β1-AR partially improved LVPdev, suggesting β2-AR have an indirect role in improving contractile function in diabetes. This is in accordance with the reduced β1-AR and increased β2-AR expression levels observed in the left ventricle of diabetic animals.
To investigate whether β2-AR-linked coronary vasodilation influenced the indirect β2-AR associated role, the β-AR responsiveness of the coronary vasculature was investigated using total myocardial flow in the isolated heart preparation. Total myocardial flow was unchanged in diabetic hearts at rest and during β-AR stimulation, as was flow impediment, though diabetic hearts at rest had an increased flow relative to the amount of cardiac work performed, which normalised at elevated work rates. In contrast the specific β1-AR stimulation did not normalise increased diabetic flow at high work rates and flow impediment was lower. Thus the β2-AR regulation of coronary vasodilation seems to have an adaptive role in preserving coronary flow during diastole and may support the indirect role of β2-AR improvement of contractile function in diabetes. This is in accordance with the observed increase in phosphorylation of the β2-AR-Gi linked downstream kinase adenosine monophosphate-activated protein kinase (AMPK), an important cellular fuel gauge.
Thus, the results from this study suggest that changes in β-AR subtype responsiveness in diabetes cause cardiovascular and myocardial dysfunction. Critically the lower intrinsic HR is largely β-AR independent and sympathetic neural input to the heart increases β1-AR chronotropic responsiveness. The results also support the emerging role of β2-AR as a metabolic regulator that indirectly improves cardiac function in diabetes and provide new insights into the divergent role of β2-AR in cardiomyocytes away from the classical contractile pathway of β1-AR.