The development of tolerance to antilipolytic agents by isolated rat adipocytes

Comparative study on the in vivo and in vitro antilipolytic effects of etofibrate, nicotinic acid and clofibrate in the rat

The release of both glycerol and free fatty acids (FFA) into a medium by epididymal fat pad pieces from fed rats incubated in Krebs Ringer bicarbonate-albumin buffer supplemented or not with epinephrine decreased more in the presence of etofibrate than in the presence of equimolecular doses of nicotinic acid or clofibrate. The first drug was the only one to stimulate the rate of fatty acid re-esterification when incubations were done under basal conditions. By 3 h after their acute oral administration all three drugs decreased plasma FFA levels, although the effect from etofibrate was largest, the drugs enhanced or decreased plasma glycerol levels depending on both the dose and the time after treatment. Plasma triglycerides also decreased at 3 h after oral drug administration, and this effect was similar with etofibrate and nicotinic acid but less with clofibrate. With the exception of a decrease at 7 h after the highest dose (1.2 mmol/kg) of either etofibrate or nicotinic acid (but not clofibrate), plasma cholesterol levels remained stable at 7 h after the respective treatments. Thus, the hypocholesterolemic effect of these drugs seems secondary to their hypotriglyceridemic effect, which would be a consequence of their respective antilipolytic actions, and follows an efficiency sequence of etofibrate, nicotinic acid and clofibrate.

The effects of GR79236 on plasma fatty acid concentrations, heart rate and blood pressure in the conscious rat

GR79236 (N-[(1S,trans)-2-hydroxycyclopentyl]adenosine) is an orally active adenosine A1 receptor agonist, which decreases plasma non-esterified fatty acid levels in fasted rats. This study has quantified the effects of GR79236 on plasma non-esterified fatty acid levels, blood pressure and heart rate in conscious rats. Blood pressure and heart rate were continuously recorded in rats for 30 min before and 1 h after oral dosing with GR79236 (0.03-3 mg/kg) or vehicle. Plasma non-esterified fatty acid concentrations were determined in blood sampled before and 1 h after dosing. GR79236 produced dose-related reductions in plasma non-esterified fatty acid concentrations, with an almost maximal effect (63.2%) occurring after 1 mg/kg. This dose induced a small but significant decrease in heart rate (12.0%), and a non-significant decrease in mean blood pressure (6.3%). Thus, in the conscious rat, GR79236 can exert profound antilipolytic effects with minimal effects on blood pressure and heart rate.

Functional characterization of three adenosine receptor types

1. The purpose of the present study was to classify adenosine receptors into A1 and A2 subtypes in a wide range of isolated tissues and cell types (rat adipocytes and atria, guinea-pig ileum and atria (A1); guinea-pig aorta, dog coronary artery and human platelets and neutrophils (A2)) using the R- and S-diastereoisomers of N-phenylisopropyladenosine (PIA), N-cyclopentyladenosine (CPA), the novel compound, N-[(1S,trans)-2-hydroxycyclopentyl]adenosine (GR79236), N-[(2-methylphenyl)methyl]adenosine (metrifudil), 2-(phenylamino)adenosine (CV1808), and 2[[2-[4-(2-carboxyethyl)phenyl]ethyl]amino]-N- ethylcarboxamidoadenosine (CGS21680); N-ethylcarboxamidoadenosine (NECA) was used as a standard. 2. Results obtained in all tissue preparations previously reported to contain A1-receptors could be described by a single rank order of agonist potency: CPA > or = GR79236, R-PIA > or = NECA >> S-PAI > or = metrifudil > or = CV1808, CGS21680. 3. In contrast, two distinct rank orders of agonist potency were observed in preparations previously reported to contain A2-receptors. In dog coronary artery, human neutrophils and platelets the rank order of potency was: CV1808, CGS21680 > or = NECA > R-PIA > or = metrifudil > or = CPA > GR79236 S-PIA. However, in guinea-pig aorta the rank order was: NECA > metrifudil > R-PIA, CPA > CV1808, GR79236 > or = S-PIA, CGS21680. 4. The results of this study are consistent with the existence of three types of adenosine receptor: A1-and two subtypes of A2-receptor. The receptor present in dog coronary artery, human platelets and neutrophils, probably corresponds to the A2a subtype, whilst that present in the guinea-pig aorta may be of the A2b subtype.

Prostanoid-induced inhibition of lipolysis in rat isolated adipocytes: probable involvement of EP3 receptors

Sulprostone, enprostil and 16, 16 dimethyl PGE2 have all been found to be potent inhibitors of lipolysis induced by 3-isobutyl-1-methyl-xanthine (IBMX) in rat isolated adipocytes. The potency of sulprostone and enprostil in particular indicates that the response is likely to be mediated through either EP3 or EP1-receptors. However, the EP1-receptor blocking drug, AH6809, had no effect on the antilipolytic response to either PGE2 or sulprostone. We therefore conclude that the receptors mediating prostanoid-induced inhibition of lipolysis in rat adipocytes must principally be of the EP3 sub-type.

Do adenosine A3 receptors exist?

1. It has been suggested that adenosine A1 receptors may be sub-divided into A1 and A3 types, based on the relative potencies of 5'-N-ethylcarboxamidoadenosine (NECA) and selected N6-substituted adenosine analogues. At A1 receptors (rat adipocytes) N6-phenylisopropyladenosine (PIA) was reported to be approx. 100-fold more potent than NECA, whereas the compounds were equipotent at A3 receptors (those in cardiac and neuronal preparations). 2. The study reported here has systematically evaluated this proposal, the rank orders of potency of NECA, R- and S-PIA, N6-cyclopentyladenosine (CPA) and N6-cyclohexyladenosine (CHA) being determined in rat adipocytes, guinea-pig ileum and rat and guinea-pig atria. 3. R-PIA, CHA and CPA were found to have consistent potencies relative to NECA across all 6 tissues, including rat adipocytes. The rank order was CPA greater than or equal to CHA, R-PIA greater than or equal to NECA greater than S-PIA. 4. We conclude that the relative potencies of these agonists do not support the concept that adenosine A1 receptors in rat adipocytes differ from those in neuronal and cardiac tissues.

Mode of action of lipid-lowering drugs

Following the recent demonstration that both cholestyramine and nicotinic acid decrease mortality from coronary heart disease, there is a new enthusiasm for hypolipidaemic therapy. The agents in current use are, however, insufficiently active or are accompanied by unacceptable side effects. An understanding of the mode of action is necessary, both to optimize treatment guidelines (e.g. regarding combination therapy or use in specific subsets of patients) and to develop new agents with preferred actions on rate-limiting steps. A reduction in LDL cholesterol concentration remains the principal desired action, although an elevation in HDL may also be beneficial. The main categories of commercially available agent comprise the anion exchange resins (inhibitors of bile acid absorption); cholesterol absorption inhibitors; fibrates (probably acting by enhancing lipoprotein lipase); and probucol (affecting LDL clearance). The most interesting of the new agents in clinical trials are the beta-hydroxy-beta-methylglutaryl-CoA reductase inhibitors, but other types of agent are at an earlier stage of evaluation, e.g. acyl-CoA: cholesterol acyltransferase inhibitors and peptide cofactors. It is not yet certain whether all the approaches to cholesterol lowering have equal validity, although an effect on biological endpoints is obtained for a variety of agents. Future evaluation will be aided by the implementation of noninvasive methods to quantify atherosclerosis and by the use of simple, 'dry-chemistry', cholesterol assays to screen populations.