AJP - Gastrointestinal and Liver Physiology, Vol 247, Issue 1 62-G69, Copyright © 1984 by American Physiological Society
Binding of somatostatin to pancreatic acinar cells
J. P. Esteve, C. Susini, N. Vaysse, H. Antoniotti, E. Wunsch, G. Berthon and A. Ribet
The binding of 125I-[Tyr11]somatostatin to guinea pig pancreatic acini was
saturable and temperature, protein, and radioligand concentration
dependent. Dissociation rate was very slow (t1/2 = 193 +/- 24 min).
Scatchard analysis revealed a single class of binding sites with a Kd of
0.28 +/- 0.02 nM and a maximal binding capacity of 72 +/- 10.6 fmol/mg
prot. There was a strong correlation between binding capacity and
extracellular calcium concentration. Incubating acini in EGTA-containing
medium with no added Ca2+ caused a 50% decrease in maximal binding capacity
with a decrease in receptor affinity. Furthermore, in the absence of
calcium, bound somatostatin was rapidly released (t1/2 = 14 +/- 1 min).
Subcellular fractionation studies and acid treatment of acini incubated
with the tracer showed that most of the somatostatin binding sites were
located on the cell surface. Agents that altered cellular calcium in
pancreatic acini, such as analogues of cholecystokinin and cholinergic
agents, also inhibited the binding of 125I-[Tyr11]somatostatin by a
calcium-dependent process. We conclude that somatostatin binds to specific
plasma membrane receptors to form a slowly reversible complex that is
highly reactive with calcium. Cell calcium-mobilizing agents decrease the
affinity of acinar somatostatin receptors for somatostatin.
