Species- and temperature-dependency of the decrease in myofilament Ca2+ sensitivity induced by beta-adrenergic stimulation

Physiological Temperature Changes Fine-Tune 2- Adrenergic Receptor-Induced Cytosolic cAMP Accumulation

Norepinephrine (NE) controls many vital body functions by activating adrenergic receptors (ARs). Average core body temperature (CBT) in mice is 37°C. Of note, CBT fluctuates between 36 and 38°C within 24 hours, but little is known about the effects of CBT changes on the pharmacodynamics of NE. Here, we used Peltier element-controlled incubators and challenged murine hypothalamic mHypoA -2/10 cells with temperature changes of ±1°C. We observed enhanced NE-induced activation of a cAMP-dependent luciferase reporter at 36 compared with 38°C. mRNA analysis and subtype specific antagonists revealed that NE activates β 2- and β 3-AR in mHypoA-2/10 cells. Agonist binding to the β 2-AR was temperature insensitive, but measurements of cytosolic cAMP accumulation revealed an increase in efficacy of 45% ± 27% for NE and of 62% ± 33% for the β 2-AR-selective agonist salmeterol at 36°C. When monitoring NE-promoted cAMP efflux, we observed an increase in the absolute efflux at 36°C. However, the ratio of exported to cytosolic accumulated cAMP is higher at 38°C. We also stimulated cells with NE at 37°C and measured cAMP degradation at 36 and 38°C afterward. We observed increased cAMP degradation at 38°C, indicating enhanced phosphodiesterase activity at higher temperatures. In line with these data, NE-induced activation of the thyreoliberin promoter was found to be enhanced at 36°C. Overall, we show that physiologic temperature changes fine-tune NE-induced cAMP signaling in hypothalamic cells via β 2-AR by modulating cAMP degradation and the ratio of intra- and extracellular cAMP. SIGNIFICANCE STATEMENT: Increasing cytosolic cAMP levels by activation of G protein-coupled receptors (GPCR) such as the β 2-adrenergic receptor (AR) is essential for many body functions. Changes in core body temperature are fundamental and universal factors of mammalian life. This study provides the first data linking physiologically relevant temperature fluctuations to β 2-AR-induced cAMP signaling, highlighting a so far unappreciated role of body temperature as a modulator of the prototypic class A GPCR.

Impact of diabetes mellitus on the contractile properties of the left and right atrial myofilaments

OBJECTIVES

The incidence of diabetes mellitus in patients with ischaemic cardiomyopathy is increasing. To evaluate the impact of diabetes mellitus on contractility, we examined the calcium-induced force in left and right atrial myofilaments of patients with and without diabetes.

METHODS

We included 149 patients (106 without diabetes, 43 with diabetes), scheduled for elective coronary artery bypass grafting from August 2016 to June 2017. The left and right atria were excised and prepared for skinned fibre measurements (pCa-force curve). The unit for the force measurements is Millinewton (mN). Comprehensive demographic data as well as echocardiographic findings of the patients were collected.

RESULTS

We observed a significant decrease of left atrial force values in patients with diabetes, averaged over all calcium concentrations (patients with diabetes 0.50 ± 0.19 mN vs 0.68 ± 0.23 mN in patients without diabetes, P = 0.002) as well as in right atrial fibres (patients with diabetes 0.35 ± 0.17 mN vs 0.47 ± 0.21 mN in patients without diabetes, P = 0.005). There was a significant influence of repeated measurements (of the calcium concentrations) on force in left atrial myofilaments (P < 0.001). There was also a significant impact of diabetes on the force values of the different calcium concentrations in left atrial myofilaments (P 0.002). In right atrial myofilaments we also found a significant influence of repeated measurements (of the calcium concentrations) on force (P < 0.001). Additionally the impact of diabetes on the force values was significant (P = 0.005).

CONCLUSIONS

We demonstrated that diabetes mellitus has a significantly negative impact on calcium-induced force development in left and right atrial myofilaments.