The Insulin-like Growth Factor (IGF)-I E-Peptides Modulate Cell Entry of the Mature IGF-I Protein

Insulin-like growth factor (IGF)-I is a critical protein for cell development and growth. Alternative splicing of the igf1 gene gives rise to multiple isoforms. In rodents, proIGF-IA and proIGF-IB have different carboxy-terminal extensions called the E-peptides (EA and EB) and upon further posttranslational processing, produce the identical mature IGF-I protein. Rodent EB has been reported to have mitogenic and motogenic effects independent of IGF-I. However, effects of EA or EB on mature IGF-I, or whether proIGF-IA and proIGF-IB have different properties, have not been addressed. To determine whether the presence of EA or EB affected the distribution and stability of mature IGF-I protein, transient transfections of cDNAs encoding murine IGF-IA, IGF-IB, and mature IGF-I were performed in C2C12 cells, a skeletal muscle cell line. IGF-I secretion was measured by enzyme-linked immunosorbent assay of the media, and did not differ between expression of proIGF-IA, proIGF-IB, or mature IGF-I expression. Next, epitope-tagged constructs were transfected to determine cellular distribution of IGF-I, EA, and EB in the cells throughout the culture. IGF-I was detected in significantly fewer nontransfected cells in cultures transfected with mature IGF-I compared with transfection of proIGF-IA or proIGF-IB. These results demonstrate that EA and EB are not required for IGF-I secretion but that they increase cell entry of IGF-I from the media. This study provides evidence that the EA and EB may modulate IGF-I in addition to having independent activity.     

DURATION OF STEREOTYPED UNDERWATER VOCAL DISPLAYS BY MALE ATLANTIC WALRUSES IN RELATION TO AEROBIC DIVE LIMIT

During the breeding season from January to mid-April, adult male Atlantic walruses (Odobenus rosmarus rosmarus) dive repeatedly for an average duration of 4–6 min and give stereotyped underwater vocal displays. Between dives, they surface for 1–2 min, take 4–6 breaths, and give stereotyped vocalizations between breaths. Male walruses vocalize in the presence of groups of females and calves, young adult males, or by themselves as lone singers. This pattern is repeated throughout the breeding season and can be maintained for extended periods, sometimes exceeding 48 h. The prolonged underwater vocal displays of male walruses seem possible because the animals do not exceed the aerobic dive limit (ADL), estimated to be 9.8 min for a 1,100-kg animal, nor do they exceed the behavioral ADL of 7.9 min, determined from the histogram of dives for males singing alone. The number of breaths taken after dives and the postdive surface times remained fairly constant despite dive duration, suggesting that the walruses remained within their aerobic dive limits. The duration of most dives made by displaying males vocalizing alone during the breeding season, and dive duration of walruses feeding for protracted periods outside the breeding season, are both roughly half the estimated ADL.