Moderate but not severe hypothermia increases intracellular cyclic AMP through preserved production and reduced elimination

Rewarming from accidental hypothermia could be complicated by acute cardiac dysfunction but providing supportive pharmacotherapy at low core temperatures is challenging. Several pharmacological strategies aim to improve cardiovascular function by increasing cAMP in cardiomyocytes as well as cAMP and cGMP levels in vascular smooth muscle, but it is not clear what effects temperature has on cellular elimination of cAMP and cGMP. We therefore studied the effects of differential temperatures from normothermia to deep hypothermia (37 °C-20 °C) on cAMP levels in embryonic H9c2 cardiac cells and elimination of cAMP and cGMP by PDE-enzymes and ABC-transporter proteins. Our experiments showed significant elevation of intracellular cAMP in H9c2-cells at 30 °C but not 20 °C. Elimination of both cAMP and cGMP through ABC transport-proteins and PDE-enzymes showed a temperature dependent reduction. Accordingly, the increased cardiomyocyte cAMP-levels during moderate hypothermia appears an effect of preserved production and reduced elimination at 30 °C. This correlates with earlier in vivo findings of a positive inotropic effect of moderate hypothermia.

Pharmacodynamic properties for inhibition of cAMP- and cGMP elimination by pentoxifylline remain unaltered in vitro during hypothermia

BACKGROUND

Rewarming from hypothermia is associated with severe complications, one of which is hypothermia-induced cardiac dysfunction. This condition is characterized by decreased cardiac output accompanied by increased total peripheral resistance. This contributes to mortality rate approaching 40%. Despite this, no pharmacological interventions are recommended for these patients below 30 °C. Raising the intracellular levels of cAMP and/or cGMP, through PDE3- and PDE5-inhibitors respectively, have showed the ability to alleviate hypothermia-induced cardiac dysfunction in vivo. Drugs that raise levels of both cAMP and cGMP could therefore prove beneficial in patients suffering from hypothermia-induced cardiac dysfunction.

METHODS

The unselective PDE-inhibitor pentoxifylline was investigated to determine its ability to reach the intracellular space, inhibit PDE3 and PDE5 and inhibit cellular efflux of cAMP and cGMP at temperatures 37, 34, 30, 28, 24 and 20 °C. Recombinant human PDE-enzymes and human erythrocytes were used in the experiments. IC₅₀-values were calculated at all temperatures to determine temperature-dependent changes.

RESULTS

At 20 °C, the IC₅₀-value for PDE5-mediated enzymatic breakdown of cGMP was significantly increased compared to normothermia (IC₅₀: 39.4 µM ± 10.9 µM vs. 7.70 µM ± 0.265 µM, p-value = 0.011). No other significant changes in IC₅₀-values were observed during hypothermia.

CONCLUSIONS

This study shows that pentoxifylline has minimal temperature-dependent pharmacodynamic changes, and that it can inhibit elimination of both cAMP and cGMP at low temperatures. This can potentially be effective treatment of hypothermia-induced cardiac dysfunction.

TRIAL REGISTRATION

Not applicable.

Treatment of Cardiovascular Dysfunction with PDE3-Inhibitors in Moderate and Severe Hypothermia—Effects on Cellular Elimination of Cyclic Adenosine Monophosphate and Cyclic Guanosine Monophosphate

Introduction: Rewarming from accidental hypothermia is often complicated by hypothermia-induced cardiovascular dysfunction, which could lead to shock. Current guidelines do not recommend any pharmacological treatment at core temperatures below 30°C, due to lack of knowledge. However, previous in vivo studies have shown promising results when using phosphodiesterase 3 (PDE3) inhibitors, which possess the combined effects of supporting cardiac function and alleviating the peripheral vascular resistance through changes in cyclic nucleotide levels. This study therefore aims to investigate whether PDE3 inhibitors milrinone, amrinone, and levosimendan are able to modulate cyclic nucleotide regulation in hypothermic settings.

Materials and methods: The effect of PDE3 inhibitors were studied by using recombinant phosphodiesterase enzymes and inverted erythrocyte membranes at six different temperatures—37°C, 34°C, 32°C, 28°C, 24°C, and 20°C- in order to evaluate the degree of enzymatic degradation, as well as measuring cellular efflux of both cAMP and cGMP. The resulting dose-response curves at every temperature were used to calculate IC₅₀ and Ki values.

Results: Milrinone IC₅₀ and Ki values for cGMP efflux were significantly lower at 24°C (IC₅₀: 8.62 ± 2.69 µM) and 20°C (IC₅₀: 7.35 ± 3.51 µM), compared to 37°C (IC₅₀: 22.84 ± 1.52 µM). There were no significant changes in IC₅₀ and Ki values for enzymatic breakdown of cAMP and cGMP.

Conclusion: Milrinone, amrinone and levosimendan, were all able to suppress enzymatic degradation and inhibit extrusion of cGMP and cAMP below 30°C. Our results show that these drugs have preserved effect on their target molecules during hypothermia, indicating that they could provide an important treatment option for hypothermia-induced cardiac dysfunction.

Inhibition of ABCC5-mediated cGMP transport by progesterone, testosterone and their analogues

The biodynamics and biokinetics of sex hormones are complex. In addition to the classical steroid receptors (nuclear receptors), these hormones act through several non-genomic mechanisms. Modulation of ABC-transporters by progesterone represents a non-genomic mechanism. In the present study, we employed inside out vesicles from human erythrocytes to characterize high affinity cGMP transport by ABCC5 (member 5 of the ATP-Binding Cassette subfamily C). Progesterone and testosterone inhibited the transport with respective K_i of 1.2 ± 0.3 and 2.0 ± 0.6 μmol/L. We used virtual ligand screening (VLS) to identify analogues to progesterone and testosterone. A large number of substances were screened in silico and the 19 most promising candidates were screened in vitro. Each substance was tested for a concentration of 10 μmol/L. The range of cGMP transport reduction was 21.5% to 86.2% for progesterone analogues and 8.6% to 93.8 % for testosterone analogues. Three of the most potent test compounds (TC) of each analogue class, in addition to progesterone and testosterone, were characterized for concentrations from 1 nanomol/L to 1 mmol/L. The progesterone analogues showed following K_i-values (μmol/L): TC-08: 0.61, TC-16: 0.66 and TC-15: 9.3. The K_i-values (μmol/L) for the testosterone analogues were: TC-18: 0.10, TC-07: 0.67 andTC-05: 2.0. The present study shows that VLS may be a versatile tool in the development of membrane transport modulating agents (MTMAs).

Treatment of Cardiovascular Dysfunction With PDE5-Inhibitors - Temperature Dependent Effects on Transport and Metabolism of cAMP and cGMP

Introduction

Cardiovascular dysfunction is a potentially lethal complication of hypothermia. Due to a knowledge gap, pharmacological interventions are not recommended at core temperatures below 30°C. Yet, further cooling is induced in surgical procedures and survival of accidental hypothermia is reported after rewarming from below 15°C, advocating a need for evidence-based treatment guidelines. In vivo studies have proposed vasodilation and afterload reduction through arteriole smooth muscle cGMP-elevation as a favorable strategy to prevent cardiovascular dysfunction in hypothermia. Further development of treatment guidelines demand information about temperature-dependent changes in pharmacological effects of clinically relevant vasodilators.

Materials and Methods

Human phosphodiesterase-enzymes and inverted erythrocytes were utilized to evaluate how vasodilators sildenafil and vardenafil affected cellular efflux and enzymatic breakdown of cAMP and cGMP, at 37°C, 34°C, 32°C, 28°C, 24°C, and 20°C. The ability of both drugs to reach their cytosolic site of action was assessed at the same temperatures. IC₅₀- and K_i-values were calculated from dose–response curves at all temperatures, to evaluate temperature-dependent effects of both drugs.

Results

Both drugs were able to reach the intracellular space at all hypothermic temperatures, with no reduction compared to normothermia. Sildenafil IC₅₀ and K_i-values increased during hypothermia for enzymatic breakdown of both cAMP (IC₅₀: 122 ± 18.9 μM at 37°C vs. 269 ± 14.7 μM at 20°C, p < 0.05) and cGMP (IC₅₀: 0.009 ± 0.000 μM at 37°C vs. 0.024 ± 0.004 μM at 32°C, p < 0.05), while no significant changes were detected for vardenafil. Neither of the drugs showed significant hypothermia-induced changes in IC₅₀ and K_i–values for inhibition of cellular cAMP and cGMP efflux.

Conclusion

Sildenafil and particularly vardenafil were ableto inhibit elimination of cGMP down to 20°C. As the cellular effects of these drugs can cause afterload reduction, they show potential in treating cardiovascular dysfunction during hypothermia. As in normothermia, both drugs showed higher selectivity for inhibition of cGMP-elimination than cAMP-elimination at low core temperatures, indicating that risk for cardiotoxic side effects is not increased by hypothermia.

Expression of nuclear progesterone receptors (nPRs), membrane progesterone receptors (mPRs) and progesterone receptor membrane components (PGRMCs) in the human endometrium after 6 months levonorgestrel low dose intrauterine therapy

The classical steroid receptors (nuclear receptors), including those for progesterone (nPRs), are thoroughly characterized. The knowledge about so-called non-genomic effects, which are mediated by extra-nuclear initiated signals, has increased immensely the last decades. In a previous clinical study of endometrial hyperplasia, we observed that the antiproliferative progestin effect persisted after 3 months treatment with levonorgestrel (LNG) intrauterine system (IUS) even with a complete downregulation of nPRs. This raised the question of what other mechanisms than signaling through nPRs could explain such an observation. In the present study, RT-qPCR was employed to characterize mRNA expression for nPRs, membrane progesterone receptors (mPRs) and progesterone receptor membrane components (PGRMCs) in women (n = 42) with endometrial hyperplasia that received intrauterine low dose LNG for 6 months. At the end of this period endometrial tissue showed that nPRs were virtually completely downregulated (≈ 10 % of baseline) whereas the levels of remaining mPRs, subtype-α, -β and -γ were 76 %, 59 % and 73 % of baseline, respectively. PGRMC1 was downregulated to 15 % of baseline, in contrast to PGRMC2, which was upregulated to about 30 % above baseline. We used human cancer cells from uterine cervix (C-4I cells) as control. Progesterone caused a concentration-dependent antiproliferative effect but in several and separate studies, we were unable to detect nPRs (immunocytochemistry) in the C-4I cells. The use of RT-qPCR showed that nPRs were undetectable in C-4I cells, in contrast to mPRs and PGRMCs with a distinct mRNA expression. The present study suggests that mPRs and/or PGRMCs preserve the antiproliferative effect of LNG in the human endometrium and are responsible for the concentration-dependent antiproliferative effect of progesterone in C-4I cells.

Data on RT-qPCR assay of nuclear progesterone receptors (nPR), membrane progesterone receptors (mPR) and progesterone receptor membrane components (PGRMC) from human uterine endometrial tissue and cancer cells of the Uterine Cervix

A previous investigation showed that the endometrium normalized in women with endometrial hyperplasia after three months treatment with high dose levonorgestrel IUS (intrauterine system) [1] . The effect was maintained even if immunohistochemical analyses of the endometrium showed that nuclear progesterone receptors (nPRs) were completely downregulated. These observations indicated that some type of non-genomic effect existed [2]. We conducted new investigations of endometrial hyperplasia, now with 6 months low dose levonorgestrel IUS treatment. Again, the growth disturbances were reversed with normalization of the endometrium [3,4]. In the context of these studies, RT-qPCR analyses of the endometrium were performed before and after treatment, to determine expression of nuclear progesterone receptors (nPRA+B and nPRB), membrane progesterone receptors (mPR, α-, β- and γ-subtypes) and progesterone receptor membrane components (PGRMC1and PGRMC2). The human cervical cell line (C-4 I) [5] with no detectable nPRs [6,7] , was included in the investigation as biological control .The gene expression of nPRs, mPRs and PGRMCs was determined in the logarithmic growth phase. Tissue and cellular mRNA was determined with RT-qPCR and used as a surrogate marker for receptor (protein) expression. The present data are connected to the related article entitled “Expression of nuclear progesterone receptors (nPRs), membrane progesterone receptors (mPRs) and progesterone receptor membrane components (PGRMCs) in the human endometrium after 6 months levonorgestrel low dose intrauterine therapy” [8].

The role of OAT2 (SLC22A7) in the cyclic nucleotide biokinetics of human erythrocytes

The present study was conducted to characterise the transporter(s) responsible for the uptake of cyclic nucleotides to human erythrocytes. Western blotting showed that hRBC expressed OAT2 (SLC22A7), but detection of OAT1 (SLC22A6), or OAT3 (SLC22A8) was not possible. Intact hRBC were employed to clarify the simultaneous cyclic nucleotide egression and uptake. Both these opposing processes were studied. The K_m‐values for high affinity efflux was 3.5 ± 0.1 and 39.4 ± 5.7 μM for cGMP and cAMP, respectively. The respective values for low affinity efflux were 212 ± 11 and 339 ± 42 μM. The uptake was characterised with apparently low affinity and similar K_m‐values for cGMP (2.2 mM) and cAMP (0.89 mM). Using an iterative approach in order to balance uptake with efflux, the predicted real K_m‐values for uptake were 100–200 μM for cGMP and 50–150 μM for cAMP. The established OAT2‐substrate indomethacin showed a competitive interaction with cyclic nucleotide uptake. Creatinine, also an OAT2 substrate, showed saturable uptake with a K_m of 854 ± 98 μM. Unexpectedly, co‐incubation with cyclic nucleotides showed an uncompetitive inhibition. The observed K_m‐values were 399 ± 44 and 259 ± 30 μM for creatinine, in the presence of cGMP and cAMP, respectively. Finally, the OAT1‐substrate para‐aminohippurate (PAH) showed some uptake (K_m‐value of 2.0 ± 0.4 mM) but did not interact with cyclic nucleotide or indomethacin transport.