The pharmacokinetics of colistin was investigated using specific high-performance liquid chromatography (HPLC) to measure the concentrations of colistin and colistin A and B in plasma and urine in five rats after administration of an intravenous bolus of 1 mg of colistin sulfate/kg of body weight. There were differences in the pharmacokinetic behaviors of unbound colistin A and B. This is the first report of the use of HPLC to study the pharmacokinetics of colistin and its two major components.

Colistin appeared in plasma soon after administration of CMS, indicating rapid conversion of CMS into colistin. CMS had a shorter terminal half-life than did colistin, indicating that the disposition of the colistin generated from CMS was rate-limited by its elimination. Most of the dose was recovered in urine, half in the form of colistin. The high percentage of colistin recovered in urine was believed to be formed by hydrolysis of CMS in the bladder and in the collection vessel, and/or conversion from CMS in the kidney.

The growth and atresia of individual Graafian follicles in the ovaries of fourteen cycling Suffolk ewes were followed in vivo by marking the large follicle(s) present in the ovaries with Indian ink at three different times during the oestrous cycle. The ovaries containing marked follicles were removed on the 3rd to 9th day of the next cycle and sectioned to determine whether the follicles had ovulated. The results showed that follicles marked before the last day of the cycle regressed and did not ovulate. Only large follicles marked at, or just before, the onset of oestrus, ovulated and developed into corpora lutea. It is suggested that there are at least three phases of follicular growth and atresia during any one oestrous cycle which appear to be independent of circulating progesterone levels. Examination of sections of ovaries from a number of anoestrous and pregnant ewes gave no indication of a similar cyclic pattern of follicular growth and atresia.

No abstract

The concentration of unconjugated oestrone, oestradiol-17a and oestradiol-17/? has been measured in the maternal jugular venous plasma of pregnant sheep every 6 hr over the period 108 hr before to 72 hr after parturition. Measurable levels of each oestrogen were present 108 hr before parturition and by about 40 hr, the concentration began to increase markedly. Levels of 200 to 350 pg/ml for oestrone and 100 to 150 pg/ml for oestradiol-17a and for oestradiol-17/? were found just before parturition. At 12 hr after parturition, the concentration of oestrone and oestradiol-17ß had fallen to < 15 pg/ml. Levels ofoestradiol17a remained elevated for a longer period after parturition and did not fall to <35 pg/ml until 48 to 60 hr after parturition.