Measurement of pulsatile insulin secretion in the rat: direct sampling from the hepatic portal vein

Abstract

It has previously been shown that insulin is secreted in discrete secretory bursts by sampling directly from the portal vein in the dog and humans. Deficient pulsatile insulin secretion is the basis for impaired insulin secretion in type 2 diabetes. However, while novel genetically modified disease models of diabetes are being developed in rodents, no validated method for quantifying pulsatile insulin secretion has been established for rodents. To address this we 1) developed a novel rat model with chronically implanted portal vein catheters, 2) established the parameters to permit deconvolution of portal vein insulin concentrations profiles to measure insulin secretion and resolve its pulsatile components, and 3) measured total and pulsatile insulin secretion compared with that in the dog, the species in which this sampling and deconvolution approach was validated for quantifying pulsatile insulin secretion. In rats, portal vein catheter patency and function were maintained for periods up to 2-3 wk with no postoperative complications such as catheter tract infection. Rat portal vein insulin concentration profiles in the fasting state revealed distinct insulin oscillations with a periodicity of approximately 5 min and an amplitude of up to 600 pmol/l, which was remarkably similar to that in the dogs and in humans. Deconvolution analysis of portal vein insulin concentrations revealed that the majority of insulin ( approximately 70%) in the rat is secreted in distinct insulin pulses occurring at approximately 5-min intervals. This model therefore permits direct accurate measurements of pulsatile insulin secretion in a relatively inexpensive animal. With increased introduction of genetically modified rat models will be an important tool in elucidating the underlying mechanisms of impaired pulsatile insulin secretion in diabetes.

Fig. 1.

Plasma insulin concentrations in rats at the portal vein (left) and carotid artery (right) after a bolus of insulin (5 mU) was injected into the inferior mesenteric vein at the time (0 min) during somatostatin (10 μg·kg⁻¹·min⁻¹) infusion to ablate endogenous insulin secretion. The decay curve of insulin concentrations was biexponential at the portal vein and monoexponential at the arterial sampling site. The mean calculated mono- and biexponential half-lives are shown on the graph.

Fig. 2.

Minute-by-minute sampled insulin concentration profiles in the portal vein (•) and in the carotid artery (○) in 3 representative adult Sprague-Dawley rats (left) and 3 representative adult Mongrel dogs (right).

Fig. 3.

Insulin secretion rates calculated (by deconvolution analysis of portal vein insulin concentrations) at fasting in 3 representative adult Sprague-Dawley rats (left) and 3 representative adult Mongrel dogs (right).

Fig. 4.

Mean percentage of insulin derived from insulin secretory bursts (A), mean insulin interpulse interval (B), mean deconvolved insulin secretion (C), and insulin burst mass (D) in 7 adult Sprague-Dawley rats and 7 adult Mongrel dogs at fasting. *P < 0.05, **P < 0.01 for dog vs. rat.

Fig. 5.

Representative portal vein insulin concentrations profiles (black) in a rat (top left) and a dog (top right) together with deconvolution defined best-fit curves (gray) to quantify endogenous insulin secretion shown at bottom. Insulin concentrations are expressed in pmol/l ,and insulin secretion rates are expressed in nmol·l⁻¹·min⁻¹. *P < 0.05, denotes insulin peaks tested for statistical significance.