Is relaxin a calcium transporter/buffer?

Abstract

Relaxin inhibits smooth muscle contractions by binding Ca2+ and making it unavailable to the contractile elements. Rat uterus sarcoplasmic reticulum (SR) was isolated, incubated with 45Ca in the presence and absence of porcine relaxin, and filtered. In the presence of relaxin, only 4% of the 45Ca was on the filter with the SR as compared to that measured on the filter with SR alone or in the presence of proteins of similar isoelectric point (cytochrome C) and size (insulin). A Sephadex G-25 column was equilibrated with buffer containing 45Ca, and 0.5 mL fractions were collected. Each fraction contained 240-250 K cpm. Relaxin was diluted in the buffer, added to the top of the column, and eluted. A peak of 45Ca (>600 K cpm) was found with the eluted relaxin. No peak of 45Ca was associated with cytochrome C or insulin under the same conditions. The eluted fractions containing 45Ca and relaxin were combined. A sample was incubated with rabbit anti-porcine relaxin serum and goat anti-rabbit serum and centrifuged, and 45Ca in the pellet and supernatant was determined. Ninety-eight percent of the 45Ca was found in the supernatant. Thus, 45Ca was not tightly bound to relaxin or the antibodies knocked the 45Ca off. Is relaxin a calcium transporter or buffer?