Exposure of the body to microgravity during space flight causes a seri
es of well-documented changes in Ca2+ metabolism, yet the cellular and
molecular mechanisms leading to these changes are poorly understood.
Calbindins, vitamin D-dependent Ca2+ binding proteins, are believed to
have a significant role in maintaining cellular Ca2+ homeostasis. In
this study, we used biochemical and immunocytochemical approaches to a
nalyze the expression of calbindin-D-28k and calbindin-D-9k in kidneys
, small intestine, and pancreas of rats flown for 9 d aboard the space
shuttle. The effects of microgravity on calbindins in rats from space
were compared with synchronous Animal Enclosure Module controls, mode
led weightlessness animals (tail suspension), and their controls. Expo
sure to microgravity resulted in a significant and sustained decrease
in calbindin-D-28k content in the kidney and calbindin-D-9k in the sma
ll intestine of flight animals, as measured by enzyme-linked immunosor
bent assay (ELISA). Modeled weightlessness animals exhibited a similar
decrease in calbindins by ELISA. Immunocytochemistry (ICC) in combina
tion with quantitative computer image analysis was used to measure in
situ the expression of calbindins in the kidney and the small intestin
e, and the expression of insulin in pancreas. There was a large decrea
se of immunoreactivity in renal distal tubular cell-associated calbind
in-D-28k and in intestinal absorptive cell-associated calbindin-D-9k o
f space flight and modeled weightlessness animals compared with matche
d controls. No consistent difference in pancreatic insulin immunoreact
ivity between space flight, modeled weightlessness, and controls was o
bserved. Regression analysis of results obtained by quantitative ICC a
nd ELISA for space flight, modeled weightlessness animals, and their c
ontrols demonstrated a significant correlation. These findings after a
short-term exposure to microgravity or modeled weightlessness suggest
that a decreased expression of calbindins may contribute to the disor
ders of Ca2+ metabolism induced by space flight.