The role of cyclic AMP in temperature-dependent changes of contractile force and sensitivity ot isoprenaline and papaverine in guinea-pig atria

Thermosensory Roles of G Protein-Coupled Receptors and Other Cellular Factors in Animals

In this review, we introduce the concept of "dual thermosensing mechanisms," highlighting the functional collaboration between G protein-coupled receptors (GPCRs) and transient receptor potential (TRP) channels that enable sophisticated cellular thermal responsiveness. GPCRs have been implicated in thermosensory processes, with recent findings identifying several candidates across species, including mammals, fruit flies, and nematodes. In many cases, these GPCRs work in conjunction with another class of thermosensors, TRP channels, offering insights into the complex mechanisms underlying thermosensory signaling. We examine how GPCRs function as thermosensors and how their signaling regulates cellular thermosensation, illustrating the complexity of thermosensory systems. Understanding these dual thermosensory mechanisms would advance our comprehension of cellular thermosensation and its regulatory pathways.

Temperature alters the inotropic, chronotropic and proarrhythmic effects of histamine in atrial muscle preparations from humans and H2-receptor overexpressing mice

We investigated whether hypothermia and hyperthermia can alter the efficacy and potency of histamine at increasing the force of cardiac contractions in mice that overexpress the human H2 receptor only in their cardiac myocytes (labelled H2-TG). Contractile studies were performed in an organ bath on isolated, electrically driven (1 Hz) left atrial preparations and spontaneously beating right atrial preparations from H2-TG mice and wild-type (WT) littermate control mice. The basal beating rate in the right atrial preparations from H2-TG mice was lowered by hypothermia (23 °C) and elevated by hyperthermia (42 °C). Furthermore, the efficacy of histamine (0.01-100 µM) at exerting positive inotropic effects was more severely attenuated in the left and right H2-TG mouse atria under hypothermia and hyperthermia than under normothermia (37 °C). Similarly, the inotropic response to histamine was attenuated under hypothermia and hyperthermia in isolated electrically stimulated (1 Hz) right atrial preparations obtained from humans undergoing cardiac surgery. The phosphorylation state of phospholamban at serine 16 at 23 °C was inferior to that at 37 °C in left atrial preparations from H2-TG mice in the presence of 10 µM histamine. In contrast, in human atrial preparations, the phosphorylation state of phospholamban at serine 16 in the presence of 100 µM histamine was lower at 42 °C than at 37 °C. Finally, under hyperthermia, we recorded more and longer lasting arrhythmias in right atrial preparations from H2-TG mice than in those from WT mice. We conclude that the inotropic effects of histamine in H2-TG mice and in human atrial preparations, as well as the chronotropic effects of histamine in H2-TG mice, are temperature dependent. Furthermore, we observed that, even without stimulation of the H2 receptors by exogenous agonists, temperature elevation can increase arrhythmias in isolated right atrial preparations from H2-TG mice. We propose that H2 receptors play a role in hyperthermia-induced supraventricular arrhythmias in human patients.

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.

The proarrhythmic effects of hypothermia in atria isolated from 5-HT4-receptor-overexpressing mice

We investigated whether hypothermia would be arrhythmogenic in mice that overexpress the human 5-HT₄ receptor only in their cardiac myocytes (5-HT₄-TG). Contractile studies were performed in isolated, electrically driven (1 Hz) left and spontaneously beating right atrial preparations of 5-HT₄-TG and littermate wild-type control mice (WT). Hypothermia (23 °C) decreased the force of contraction in the mouse right and left atrial preparations. Moreover, the concentration-dependent positive inotropic effects of 5-HT were blunted but still shifted to lower 5-HT concentrations in the left 5-HT₄-TG atria in hypothermia compared to normothermia (37 °C). Furthermore, hypothermia increased the incidence of right atrial arrhythmias in 5-HT₄-TG more than in WT mice. In contrast, at 37 °C, lowering the potassium concentration from 5.2 to 2.0 mM also induced arrhythmias in the right atrium, but with a similar incidence in WT and 5-HT₄-TG mice. In contrast, 10 μM d,l-sotalol and 300 μM erythromycin did not induce arrhythmias. Hypothermia was accompanied by the increased expression of heat shock protein 70 (HSP70) in WT but not in 5-HT₄-TG mice. We concluded that without the stimulation of 5-HT₄-receptors by exogenous agonists, a simple temperature reduction can increase arrhythmias in 5-HT₄-TG mice. It is tempting to speculate that in human patients, 5-HT₄ receptors might contribute to potentially deadly hypothermia-induced arrhythmias.

Discontinued stimulation of cardiomyocytes provides protection against hypothermia-rewarming-induced disruption of excitation-contraction coupling

NEW FINDINGS

What is the central question of this study? Will discontinued stimulation of isolated cardiomyocytes (asystole) during hypothermia mitigate hypothermia-rewarming-induced cytosolic Ca²⁺ overload? What is the main finding and its importance? Mimicking asystole or hypothermic cardiac arrest by discontinued stimulation of cardiomyocytes during hypothermia resulted in normal contractile function after rewarming. This result suggests that asystole during severe hypothermia provides protection from hypothermia-rewarming-induced contractile dysfunction in cardiomyocytes.

ABSTRACT

After exposure of spontaneously beating hearts or electrically stimulated isolated cardiomyocytes to hypothermia-rewarming (H/R), cardiac dysfunction or alteration in excitation-contraction coupling, respectively, is a consequence. In contrast, hypothermic cardiac arrest, as routinely applied during cardiac surgery, will not impose any hazard to cardiac function after rewarming. We hypothesize that by maintaining asystole during H/R, cardiomyocytes will avoid Ca²⁺ overload attributable to the transient stimulation-evoked elevation of [Ca²⁺ ]_i and thus, H/R-induced elevation of phosphorylated cardiac troponin I and reduced Ca²⁺ sensitivity after rewarming. To test this hypothesis, the aim of the study was to determine whether discontinued electrical stimulation (to imitate hypothermic cardiac arrest) versus stimulation during 3 h of H/R prevents disruption of excitation-contraction coupling in our established cardiomyocyte H/R model. Cytosolic Ca²⁺ and the contractile response (sarcomere length shortening) were measured using an IonOptix system, and the dynamic assessment of Ca²⁺ sensitivity of contraction was conducted using a phase-loop plot. Cardiomyocytes were divided into three groups. Group 1 (time-matched control) was continuously stimulated at 0.5 Hz for 3 h at 35°C. Group 2 was continuously stimulated during H/R at 0.5 Hz, whereas in group 3 stimulation was discontinued during H/R and thus the cells remained quiescent until the resumption of stimulation after rewarming. The results demonstrate that discontinued stimulation of cardiomyocytes during H/R, imitating hypothermic cardiac arrest during cardiac surgery, provides protection against H/R-induced disruption of excitation-contraction coupling. We suggest that protective effects are caused by preventing the protein kinase A-induced elevation of phosphorylated cardiac troponin I, which is a key mechanism to reduce myofilament Ca²⁺ sensitivity of contraction.

Hypothermia abolishes hypoxia-induced hyperpermeability in brain microvessel endothelial cells

The effect of mild (32 degrees C) and deep (22 degrees C) hypothermia on hypoxia-induced hyperpermeability was examined using an in vitro model of brain derived microvascular endothelial cells (BMEC). It was shown that hypoxia-induced hyperpermeability to inulin across the BMEC monolayer was completely abolished at 32 degrees C and 22 degrees C for up to 24 h of hypoxia. During normoxia, no influence of hypothermia on BMEC monolayer permeability was observed. The hypoxia-induced decrease of the cyclic AMP level after 6 h was abolished at 32 degrees C as well as at 22 degrees C of hypoxia. But after 24 h of hypoxia, hypothermia did no longer prevent the hypoxia-induced decrease of the cAMP level, which suggests that the effect of hypothermia on hypoxia-induced hyperpermeability is not caused by maintenance of the cAMP level. Because vascular endothelial growth factor (VEGF) has been shown to be the mediator of hypoxia-induced permeability changes of BMEC via the release of nitric oxide (NO), the effect of hypothermia on the VEGF expression was evaluated. During normoxia, hypothermia did not change the VEGF expression significantly but the hypoxia-induced increase in VEGF mRNA and protein expression was completely abolished at 32 degrees C and 22 degrees C respectively. Accordingly, the hypoxia-induced increase of the cGMP level was depressed by hypothermia, which demonstrates that also the amount of NO released during hypoxia is decreased at lower temperatures. Results suggest that deep as well as mild hypothermia decreased hypoxia-induced hyperpermeability by lowering the expression of the permeability-increasing protein VEGF and with it the release of NO.

The mechanism of hypothermia-induced supersensitivity of guinea pig left atria to carbachol

Hypothermia has previously been demonstrated to induce supersensitivity (defined as a decrease in the ED50) of guinea pig left atria to the negative inotropic effect of carbachol. In the present investigation, the dissociation constant (pKA or -log KA) for carbachol, determined using benzilylcholine mustard, was found to be significantly increased at 25 degrees C compared to 37 degrees C. However, the increase in pD2 (-log ED50) for carbachol at 25 degrees C was much less than would be predicted from the increase in pKA. Increasing the extracellular Ca2+ concentration or the frequency of stimulation, both of which, like hypothermia, are believed to increase Ca2+ influx into cardiac cells, resulted in a decrease in sensitivity to carbachol. Carbachol had no significant effect on cAMP or cGMP levels at either 37 degrees C or at 25 degrees C. These results suggest that the hypothermia-induced increase in sensitivity of left atria to carbachol can be explained by an increase in the affinity of the muscarinic receptor for this agonist. However, the expression of this increased affinity appears to be limited. This may be due to a concurrent decrease in the efficacy of the carbachol muscarinic receptor complex.

Hypothermia-induced supersensitivity to adenosine for responses mediated via A1-receptors but not A2-receptors

Four isolated tissues were examined in which the responses to adenosine are mediated via either A1- or A2-receptors. The responses examined were the inhibition of cholinergic transmission of field-stimulated guinea-pig ileum (A1), inhibition of noradrenergic transmission of field-stimulated rat vas deferens (A1), inhibition of developed tension of rat paced left atria (A1) and relaxation of carbachol-contracted guinea-pig trachea (A2). Cumulative concentration-response curves for adenosine and 2-chloroadenosine were constructed at 37, 30 or 27 degrees C. When plotted as a percentage of the maximum response, the concentration-response curves were displaced to the left by cooling in the ileum, vas deferens and atria, indicative of supersensitivity. This increase in sensitivity does not arise from inhibition of uptake or deamination by cooling, since it occurs equally for adenosine and 2-chloroadenosine, the latter being immune to these processes. In contrast, the sensitivity of the trachea was not affected (2-chloroadenosine) or reduced (adenosine) by cooling. Thus responses mediated via adenosine receptors of the A1 subtype exhibit hypothermia-induced supersensitivity, whereas those mediated via A2-receptors do not. This suggests a fundamental temperature-dependent difference between the two adenosine receptor subtypes.

A fundamental temperature-dependent difference between beta-adrenoceptor agonists and antagonists

Positive inotropic and chronotropic responses of guinea-pig isolated left and right atria to sympathomimetic amines were examined at bath temperatures of 38, 30 or 25 degrees C. The concentration-response curves to isoproterenol and orciprenaline were displaced to the left by cooling, indicating hypothermia-induced supersensitivity. The affinities of isoproterenol and orciprenaline were determined as their dissociation constants (pKA) from antagonism of their responses by either the functional antagonist carbachol or Ro 03-7894 which is reported to be an irreversible beta-adrenoceptor antagonist. By both methods of calculation, the affinities of isoproterenol and orciprenaline for the beta-adrenoceptors mediating inotropic and chronotropic responses were increased by lowering the temperature. In contrast, the affinity of practolol, measured as the pA2 for competitive antagonism of the isoproterenol- and orciprenaline-induced inotropic and chronotropic responses, did not increase with cooling. Thus hypothermia-induced supersensitivity is associated with an increase in agonist affinity only, which indicates a fundamental temperature-dependent difference between agonist and antagonist interactions with the beta-adrenoceptor.

Influence of temperature on the sensitivity of the adrenoceptors in the isolated atria of guinea pigs and rats

The influence of the bath temperature on the responsiveness to sympathomimetic amines was studied with isolated guinea pig and rat atria. In electrically driven guinea pig left atria, the dose-response curve for the positive inotropic effect of isoproterenol (ISO) was shifted to the left by lowering the temperature from 36 to 24 degrees C. The positive inotropic effect of phenylephrine (PHE) in lower concentrations was attenuated by lowering the temperature. Phentolamine markedly inhibited the PHE response at 36 and 32 degrees C, whereas it produced no inhibition at 24 degrees C. Similar changes were observed with rat left atria. In guinea pig left atria, propranolol inhibited the response to PHE more effectively at 24 degrees C than 32 degrees C. With guinea pig and rat atria the dose--response curve for the positive inotropic effect of PHE in the presence of phentolamine was shifted to the left by lowering the temperature. The results suggest that lowering the temperature of the bath solution diminished the positive inotropic effect of PHE mediated by alpha-adrenoceptors and potentiated that mediated by beta-adrenoceptors.

Hypothermia-induced potentiation of histamine H2-receptor-mediated relaxation and cyclic AMP increase in the isolated mesenteric artery of the rabbit

On helically cut strips of the rabbit's mesenteric artery, a temperature decrease from 42 degrees C to 25 degrees C reduced the contractile responses to histamine. Metiamide shifted the dose-response curve of the histamine-induced contraction towards higher values at 25 degrees C, but not at 42 degrees C. Furthermore, on arterial strips contracted by phenylephrine histamine evoked a dose-dependent relaxation at 25 degrees C whereas at 42 degrees C only slight relaxing responses to histamine occurred. Metiamide was capable of preventing the relaxation induced by histamine in a competitive manner. At 25 degrees C the relaxation as produced by histamine was accompanied by increases in cyclic AMP which occurred prior to the relaxing effects. Metiamide abolished the cyclic AMP increase in response to histamine. At 42 degrees C histamine was unable to elevate the cyclic AMP content. Thus, it is concluded that a cyclic AMP-mediated relaxation due to stimulation of H2-receptors counteracts the histamine-induced contraction and reduces the contractile responses to histamine at low temperatures. In addition, clear-cut evidence exists from the present study that also on artery smooth muscle the H2-receptor-mediated responses are closely associated to cyclic AMP.