What does the future hold for diabetic dyslipidaemia?

Preliminary evidence from trials with the HMG-CoA reductase inhibitors (statins), simvastatin and pravastatin, suggests that aggressive treatment of diabetic dyslipidaemia will reduce coronary events. Questions regarding the prevention of cardiovascular events in diabetic patients are now being addressed in prospectively designed trials. The first question is, can aggressive treatment of dyslipidaemia lead to primary prevention of cardiovascular events in patients with type 2 diabetes? This is being addressed in the ongoing Atorvastatin Study for the Prevention of coronary heart disease Endpoints in NIDDM (ASPEN) and the Collaborative AtoRvastatin Diabetes Study (CARDS). These trials will randomize over 4000 patients with type 2 diabetes and no previous myocardial infarction to either atorvastatin or placebo for 4 years. The second question is, are there benefits for aggressive lipid lowering to levels below those recommended in current treatment guidelines, i.e. is lower better? Results from the recent Atorvastatin VErsus Revascularization Treatment (AVERT) trial suggest this to be the case. AVERT showed that, in stable coronary heart disease patients who had been referred for revascularization, aggressive lowering of low density lipoprotein (LDL) cholesterol with atorvastatin 80 mg/day (to a mean level of 2.0 mmol/L [77 mg/dL]) reduced the incidence of ischaemic events by 36% compared with angioplasty and usual care (which reduced LDL cholesterol to 3.1 mmol/L [119 mg/dL]). The 36% reduction in events with atorvastatin versus angioplasty and usual care trended towards significance (p=0.048). The benefits of aggressive lipid-lowering therapy are also being investigated in the ongoing Treating to New Targets (TNT) and Incremental Decrease in Endpoints through Aggressive Lipid lowering (IDEAL) trials. These studies will more closely examine the benefits of treating diabetic dyslipidaemia, and will determine how aggressively this abnormal lipid profile should be treated.

Stimulation of insulin secretion by long-chain free fatty acids. A direct pancreatic effect.

A continuous-flow centrifuge was used to infuse sodium salts of oleic, linoleic, lauric, or palmitic acid into the pancreatic artery of anesthetized dogs. In these regional perfusion studies there was no increase in FFA levels in the general circulation. Elevation of pancreatic FFA levels produced an immediate increase in pancreatic venous immunoreactive insulin (IRI). After 10 min of FFA infusion. IRI levels declined somewhat from the initial peak response but soon rose again to high levels which were then sustained until the infusion was terminated. All four long-chain FFA tested produced a similar biphasic IRI response. Clearcut increases in IRI were associated with absolute FFA levels (measured in pancreaticoduodenal venous plasma) as low as 0.6-0.8 mueq/ml and with increments over basal levels of as little as 0.4-0.5 mueq/ml. At higher levels of FFA, absolute IRI levels in the pancreatic venous effluent exceeded 1,000 muU/ml in some experiments and 5- to 10-fold increases over basal values were observed. These studies indicate that long-chain FFA, in physiological concentrations, can markedly stimulate insulin secretion by a direct effect on the pancreas. The results lend support to the concept of insulin as a hormone that is importantly involved in regulating the metabolism of all three principal classes of metabolic substrates and whose release is in turn regulated by all of them. The relative importance and precise nature of its physiologic role in the regulation of lipolysis, lipid deposition, and ketone body formation remains to be established.

Infusion of long-chain fatty acid anions by continuous-flow centrifugation.

We have developed a method for the rapid infusion into plasma of large amounts of long-chain free fatty acids (FFA). Unanesthetized dogs were connected by a peripheral artery to a closed, continuousflow centrifuge from which cells and plasma emerged in separate lines. Sodium oleate was infused directly into the plasma line before cells and plasma were recombined and returned to the animal through a peripheral vein.The centrifugation procedure itself produced only small changes in circulating levels of glucose, FFA, and electrolytes. Plasma flow rates as high as 100 ml/min could be maintained, and centrifugations of 12 hr were accomplished without complications. During centrifugation, sodium oleate was infused at rates up to 80 muEq/kg per min for 2.5 hr; the maximum molar ratio of FFA to albumin without hemolysis was 10:1. Plasma FFA levels rose rapidly after infusions were started and reached constant elevated levels within 15-20 min. Oleate infusion at 10-50 muEq/kg per min produced a rise in plasma FFA proportional to the infusion rate. The maximum increment in plasma FFA above control values was 1.66 muEq/ml. When infusions ended, plasma FFA declined rapidly to control levels. Oleate infusion at rates below 30 muEq/kg per min did not reduce levels of other plasma FFA. Infusion at high rates was accompanied by a marked fall in blood glucose. This method permits adminsitration of long-chain fatty acids in sufficient quantities to study their individual metabolic effects, and provides a new way to supply lipid calories parenterally.

Stimulation of insulin secretion by infusion of free fatty acids.

The acute elevation of plasma free fatty acid (FFA) levels by direct infusion of sodium oleate into the plasma of conscious dogs was accompanied by the rapid onset of a 2- to 12-fold increase in plasma immunoreactive insulin, and, subsequently, a marked fall in plasma glucose, even in dogs receiving intravenous glucose throughout the infusion. The magnitude of both the insulin and glucose responses correlated with the mean FFA level during infusion. A large increase in plasma insulin and fall in glucose also occurred when glycerol was infused with oleate in order to simulate endogenous lipolysis more closely. Insulin levels in pancreaticoduodenal vein blood rose markedly during oleate infusion, while plasma ketone levels rose only slightly. In contrast to the effects of oleate infusion, elevation of plasma FFA to correspondingly high levels by triolein ingestion and intravenous heparin produced only small increases in plasma insulin, which did not correlate well with the FFA level reached, and small increases in plasma glucose.The results indicate that under certain conditions elevated FFA levels may be a potent stimulus of insulin secretion. This response is modified under other conditions such as during chylomicron removal under the influence of heparin. This effect may play a role in the regulation of lipolysis and ketone formation, but determination of the exact mechanism of FFA stimulation of the pancreas and its physiological significance will require further investigation.