Expression and possible role of muscle-type carnitine palmitoyltransferaseI during sperm development in the rat

Because we had found whole testis from adult rats to be much richer in the messenger RNA for the muscle (M) than for the liver (L) form of mitochondri al carnitine palmitoyltransferase I (CPT I), we sought to determine which c ell type(s) accounts for this expression pattern and how it might relate to reproductive function. Studies with immature (14-day-old) and adult animal s included 1) Northern blot analysis of testis mRNA; 2) in situ hybridizati on with slices of testis; 3) enzyme assays for CPT I, CPT II, and carnitine acetyltransferase (CAT) in testicular germ cells and nongerm cells, togeth er with measurement of the malonyl-coenzyme A (CoA) sensitivity and affinit y for carnitine of CPT 1; 4) labeling of testicular CPT I with [H-3]etomoxi r, a covalent inhibitor of the enzyme; and 5) the response of testicular an d nontesticular CPT I to dietary etomoxir. The data established the following: 1) L-CPT I was the sole isoform detecte d in immature testis. 2) Expression of the M-CPT I gene was associated only with meiotic and postmeiotic germ cells. 3) Adult testis contains a mixtur e of the L- and M-CPT I enzymes, the L and M form dominating in extratubula r cells and spermatids, respectively. Mature epididymal spermatozoa appear to be devoid of CPT I activity while possessing abundant levels of CPT II a nd CAT. 4) Five days of dietary etomoxir treatment at a dose that resulted in essentially complete inhibition of CPT I in liver, heart, skeletal muscl e, and kidney was totally without effect on either the L- or M-type enzyme in the testis of mature rats. The data point to an important role for transient expression of M-CPT I, co upled with sustained activity of CAT, in the maturation and/or function of rat sperm. They also suggest that, at least in the case of one CPT I inhibi tor (etomoxir), the testis is unusually resistant to the agent when given o rally.