The objective of this study was to establish the developmental pattern and tissue specificity of porcine myostatin expression and to evaluate expression in skeletal muscle during circumstances in which muscle growth was altered. Northern blot analysis revealed two transcripts (1.5 and 0.8 kb). Myostatin mRNA was detected in whole fetuses at 21 and 35 days and was markedly increased ( P < 0.05) by 49 days. At birth, mRNA abundance in longissimus muscle had declined significantly ( P < 0.05) from that at day 105 of gestation and continued to decrease ( P < 0.05) to its lowest level 2 wk postnatally (4 kg body wt). Myostatin expression was higher ( P < 0.05) at 55, 107, and 162 kg body wt than at 4 kg body wt. Postnatally, myostatin mRNA was detected in skeletal muscle and mammary gland. Expression at birth was 65% higher ( P < 0.04) in longissimus muscle of low-birth-weight piglets (0.57 ± 0.052 kg body wt) vs. normal (1.37 ± 0.077 kg body wt) littermates, irrespective of gender. However, suppression of longissimus muscle growth by food deprivation (3 days) did not alter ( P > 0.15) myostatin expression in either 4- or 7-wk-old piglets. Additionally, myostatin mRNA abundance was not changed by porcine growth hormone administration in growing animals. These data indicate that myostatin expression in skeletal muscle peaks prenatally and that greater expression is associated with low birth weight. Expression in mammary gland indicates a possible role for myostatin in mammary gland development and/or lactation.

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The product of the obese gene, leptin, may be an important regulator of adiposity via its regulation of feed intake and energy metabolism. Probes were developed using the polymerase chain reaction to analyze gene expression and determine the structure of the porcine ob gene. Porcine ob was expressed in adipose tissue as a 3,100 bp mRNA. Finished pigs (136 kg) had higher (P<.01) levels of ob mRNA (per unit of 6-actin mRNA) in subcutaneous adipose tissue than did growing pigs (60 kg). Obese gene expression was not detected in tissues other than adipose depots. A genomic DNA fragment containing the ob gene was isolated from a cosmid DNA library. Sequence analysis indicates that the ob gene has three exons. A short untranslated sequence was identified as exon 1 and the amino acid coding sequence was located in the second and third exons. The gene structure, intron/exon boundaries, and the amino acid sequence was highly conserved in mammalian species. The porcine leptin amino acid sequence was 95%, 92% and 89% similar to cattle, human and mouse leptin sequences, respectively.

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