Genetic differences in glucose tolerance of sheep : effect on insulin status and carcass composition

Abstract

High dietary fat intake has been linked with human health problems. Reduction in the fatness of meat-producing animals will convey substantial health and commercial benefits, since meat is a major source of dietary fat in western nations. This thesis examines a novel approach to the reduction of fatness in sheep.

Coopworth rams were selected for fast or slow clearance of plasma glucose after an intravenous glucose tolerance test. These animals, plus randomly selected controls, were mated to an unselected population of ewes. At six months of age, ram progeny of sires with slow glucose clearance (Line 1) had significantly (p < 0.05) slower clearance (76, 93 and 76 minutes for each of the three generations) than progeny of sires with fast clearance (Line 2: 60, 80, 69 minutes). Line 1 had less subcutaneous fat (GR measurement: 8.4, 8.4, 12.6 mm) in each generation than Line 2 progeny (9.2, 8.9, 13.6 mm).

Six first generation ram progeny with extreme differences in glucose tolerance were selected from each line. Basal plasma concentration of glucose was significantly (p < 0.05) higher for Line 1 (69 mg/100 ml) than Line 2 animals (63 mg/100 ml). Basal plasma insulin concentration did not differ between the lines (7.8 µU/ml). During a glucose tolerance test, plasma insulin was significantly lower at 20 and 40 minutes after injection of glucose in Line 1 than Line 2 animals, implying a smaller release of insulin in response to injected glucose.

The clearance of radioactive ¹²⁵l-insulin was modelled to estimate insulin pool sizes and flow rates in the six extreme animals from each line. All three pools were significantly (p < 0.05) larger in Line 1 (61, 115 and 191 mU) than Line 2 animals (45, 82 and 112 mU). Flow rates between the pools tended to be greater in Line 1 than Line 2 animals, but the differences were not statistically significant.

Euglycaemic clamp experiments at three insulin infusion rates, showed that Line 1 had significantly greater glucose utilization (3, 9 and 10 mg/kg liveweight º•⁷⁵/minute) than Line 2 animals (1, 5 and 6 mg/kg lwt º•⁷⁵/min). The differences remained even when glucose clearance was corrected to the same basal glucose concentration. The increase in glucose utilization, as insulin infusion rate increased from level one to level two (insulin sensitivity), was significantly greater in Line 1 than Line 2 animals (0.68 versus 0.35 µU/kg lwt º•⁷⁵/min). The results demonstrate that Line 1 have greater glucose utilization and sensitivity to insulin than Line 2 animals.

Using Scatchard analysis, approximately ten times more insulin receptors were measured in muscle than adipose tissue in both lines of sheep. There tended to be more low affinity receptors in Line 1 (1.05 and 0.80 nM/200 ug membrane protein for muscle and adipose membranes respectively) than Line 2 animals (0.74 and 0.48 nM/200 ug), but the differences were not statistically significant. The differences in the low affinity receptors may cause the partitioning of nutrients into muscle rather than fat tissue in the Line 1 animals.

Using a human insulin receptor cDNA probe, restriction fragment length polymorphisms were detected in DNA from both lines of sheep. These polymorphisms did not appear to be correlated with the differences in insulin status between the two lines.

Selection of sheep for differences in glucose tolerance has resulted in lines of sheep that differ in insulin characteristics and carcass composition. A model is presented to explain the differences in insulin characteristics between the two lines of sheep. There are three alternatives for the location of a primary defect: the insulin receptor in muscle and adipose tissue, sensitivity of the pancreas to glucose or the basal glucose transport protein.