Effects of theophylline and dibutyryl-cAMP on adenosine receptors and heart rate in cultured cardiocytes

Adenosine, Adenosine Receptors and Neurohumoral Syncope: From Molecular Basis to Personalized Treatment

Adenosine is a ubiquitous nucleoside that is implicated in the occurrence of clinical manifestations of neuro-humoral syncope (NHS). NHS is characterized by a drop in blood pressure due to vasodepression together with cardio inhibition. These manifestations are often preceded by prodromes such as headaches, abdominal pain, feeling of discomfort or sweating. There is evidence that adenosine is implicated in NHS. Adenosine acts via four subtypes of receptors, named A1 (A1R), A2A (A2AR), A2B (A2BR) and A3 (A3R) receptors, with all subtypes belonging to G protein membrane receptors. The main effects of adenosine on the cardiovascular system occurs via the modulation of potassium ion channels (IK Ado, K ATP), voltage-gate calcium channels and via cAMP production inhibition (A1R and A3R) or, conversely, through the increased production of cAMP (A2A/BR) in target cells. However, it turns out that adenosine, via the activation of A1R, leads to bradycardia, sinus arrest or atrioventricular block, while the activation of A2AR leads to vasodilation; these same manifestations are found during episodes of syncope. The use of adenosine receptor antagonists, such as theophylline or caffeine, should be useful in the treatment of some forms of NHS. The aim of this review was to summarize the main data regarding the link between the adenosinergic system and NHS and the possible consequences on NHS treatment by means of adenosine receptor antagonists.

Leptin protects the cardiac myocyte cultures from hypoxic damage

Leptin, a circulating hormone mainly produced by adipose tissue, regulates fatty acid metabolism and causes multiple systemic biological actions even the regulation of cardiovascular function. It is previously known that leptin is a hypoxia-inducible hormone, that hypoxic conditions increase the expression of this peptide in various tissues such as placenta, pancreas and also in the heart. Since leptin receptors are present in the heart, we hypothesized that whether leptin was a protector response for tissues especially for the heart against the deleterious effects of hypoxia. Cultured cardiomyocytes from newborn rats were initially treated with 3000 ng/ml leptin incubation for 1, 5 and 20 h separately, then subjected to 120 min of hypoxia. Hypoxic damage of myocytes was assayed using the measurements of both lactate dehydrogenase and creatine kinase releases into the medium and performing morphological observations (ultrastructural and immunocytochemical) of plates. The obtained results from leptin treated and non-treated control groups were compared to each other, and these data have demonstrated that 5 h of leptin treatment before hypoxia provides a significant protection for cardiomyocytes against hypoxia. Neither 1- nor 20-h leptin treated groups exhibited sufficient protection against hypoxia. In conclusion, leptin protects the cardiomyocyte cultures from hypoxia, but this effect is selective and evident only in the 5-h treated myocytes.

Modulation of cardiac A1-adenosine receptors in rats following treatment with agents affecting heart rate

Effects of chronic treatment affecting heart rate on A1 adenosine receptor levels and their functions were studied. Treatment of rats with isoproterenol for 10 days accelerated heart rate and increased the level of adenosine receptors, in both the atria and ventricles. Negative dromotropic response of isolated heart to adenosine was enhanced in isoproterenol-treated rats. Similar results were obtained following treatment with atropine sulfate, or swimming training but not after treatment with thyroxine. On the other hand, treatment with amiodarone, which normally causes a decrease in heart rate, also increased the level of adenosine receptors in both atria and ventricles. The sensitivity of the isolated heart to the negative dromotropic and chronotropic effects of adenosine was not enhanced in the amiodarone treated rats. Similar results were obtained following treatment with propranolol, while treatment with PTU (6-n-propyl-2-thiouracil) increased adenosine sensitivity in the isolated heart. It was concluded that the levels of A1 adenosine receptors in the heart correspond to heart rate, and to cardiac efficiency. While an increase in heart rate was followed by up-regulation of A1 adenosine receptors, a decrease in heart rate caused a moderate elevation of these receptors.

Uptake of glucose analogs reflects the rate of contraction of cultured myocytes

The present study demonstrates that: a) adenosine and R-N6-(2-phenylisopropyl)-adenosine (R-PIA, A1 and A3 adenosine receptor agonist) inhibited [3H]deoxyglucose uptake or [3H]3-O-methyl-D-glucose uptake; b) sugar uptake reflects the rate of contraction in cardiac cultures; c) [3H]deoxyglucose uptake or [3H]3-O-methyl-D-glucose uptake are useful quantitative probes for beating rate evaluation. A 25-40% decrease in [3H]deoxyglucose uptake (p < 0.01) was obtained following 13-21 min treatment with 100 microM adenosine together with 1 microM dipyridamole or with 10 microM R-PIA, which inhibited spontaneous contractions. Adenosine (10 microM) attenuated spontaneous beating rate and inhibited approximately 55% of the [3H]deoxyglucose uptake following 22 h treatment (p < 0.01). 1 microM R-PIA also attenuated beating rate following either a short (1 min) or long (24 h) application and decreased [3H]deoxyglucose uptake by 20-30% (p < 0.01) during 0.5-24 h of treatment. A 157 +/- 9% and 205 +/- 11% increase (p < 0.01) in [3H]deoxyglucose uptake was obtained at 27 and 37 degrees C, respectively, compared with the uptake at 17 degrees C, which completely inhibited spontaneous contractions. Similar results [33 +/- 6% (p < 0.01) and 21 +/- 8% (p < 0.05) inhibition in [3H]deoxyglucose uptake] were obtained following 2 and 22 h of carbamylcholine treatment, respectively. This treatment also reduced spontaneous contractions. [3H] 3-O-Methyl-D-glucose uptake also decreased by 31 +/- 12% (p < 0.05) as a result of the arrest of contractions by adenosine. Elevations of 90 +/- 13% and 34 +/- 11% (p < 0.01) in [3H]deoxyglucose uptake were obtained following treatment with isoprenaline after 2 and 22 h application, respectively. It is concluded that adenosine and R-PIA inhibited [3H]deoxyglucose uptake or [3H] 3-O-methyl-D-glucose uptake in rat heart culture and that there is a linkage between the rate of cardiac contractions in culture and sugar uptake.