EFFECT OF EXOGENOUS INSULIN ON PLASMA AND FOLLICULAR INSULIN-LIKE GROWTH-FACTOR-I, INSULIN-LIKE GROWTH-FACTOR BINDING-PROTEIN ACTIVITY, FOLLICULAR ESTRADIOL AND PROGESTERONE, AND FOLLICULAR-GROWTH IN SUPEROVULATED ANGUS AND BRAHMAN COWS

Angus (n = 14) and Brahman (n = 14) cows were used to evaluate the eff ects of insulin administered concomitantly with FSH in a superovulatio n regimen. Cows were allotted to four pen replicates by treatment and breed, and received FSH (i.m.) twice a day for 5 consecutive days (fir st day of injections = day 0 of study) plus concomitant administration of either saline (control) or long-acting bovine insulin (0.25 iu kg( -1) body mass; s.c.). Blood samples were collected at intervals of 6 h during the injection period and analysed for plasma insulin, glucose, insulin-like growth factor I (IGF-I) and IGF-I binding protein (IGFBP ) activity. Cows were ovariectomized on day 5. The number and diameter of follicles were recorded. Follicular fluid was aspirated for determ ination of IGF-I, IGFBP activity, oestradiol and progesterone. Mean pl asma concentration of glucose was lower in insulin-treated than in con trol cows averaged over days 1-5 (56 +/- 3 versus 82 +/- 3 mg dl(-1); P < 0.01). Plasma concentration of IGF-I and IGFBP activity were not a ffected (P > 0.10) by treatment, but were higher in Brahman than in An gus cows (IGF-I: 41 +/- 6 versus 19 +/- 6 ng ml(-1), P < 0.05; IGFBP a ctivity: 17.5 +/- 0.4 versus 15.8 +/- 0.04% (10 mu l)(-1); P < 0.03). Insulin treatment did not affect the number of small (1.0-3.9 mm), med ium (4.0-7.9 mm) or large (greater than or equal to 8.0 mm) follicles. Brahman cows had a greater (P < 0.01) number of medium and total foll icles (19.4 +/- 2.5 and 60.5 +/- 5.5, respectively) than did Angus cow s (7.5 +/- 2.6 and 30.5 +/- 5.6, respectively). Diameter of large foll icles was greater in insulin-treated than in control cows (11.4 +/- 0. 2 versus 10.6 +/- 0.1 mm; P < 0.05). Follicular fluid IGF-I concentrat ion in large follicles was higher in insulin-treated Brahman cows (60 +/- 2 ng ml(-1)) than in control Brahman cows (37 +/- 2 ng ml(-1)), bu t was lower in insulin-treated Angus cows (31 +/- 3 ng ml(-1)) than in control Angus cows (38 +/- 2 ng ml(-1); treatment x breed interaction , P < 0.01). IGFBP activity in fluid from large follicles was not affe cted by insulin treatment in Brahman cows but was reduced (P < 0.05) b y insulin treatment in Angus cows. In large follicles, concentration o f oestradiol in follicular fluid was higher in insulin-treated than in control cows (144 +/- 36 versus 29 +/- 28 ml(-1); P < 0.10), while pr ogesterone concentration was lower in insulin-treated than in control cows (71 +/- 24 versus 178 +/- 19 ng ml(-1); P < 0.05). The percentage of large follicles that were oestrogen-active was higher (P < 0.05) i n insulin-treated cows (54 +/- 4.9%) than in control cows (25 +/- 4.5% ). In summary, insulin treatment resulted in increased follicle diamet er, higher oestradiol and lower progesterone concentrations in the flu id of large follicles, but did not increase the number of follicles. B rahman cows had higher plasma and follicular fluid concentrations of I GF-I, lower oestradiol concentrations and a greater number of follicle s than did Angus cows.