Subcellular distribution of calmodulin and its binding proteins within the rat submandibular gland

The involvement of calmodulin (CaM) in rat submandibular gland mucin secretion was investigated in vitro using a dispersed cell preparation. The CaM antagonist trifluoperazine (TFP) inhibited mucin secretion in response to both isoproterenol and dibutyryladenosine 3',5'-cyclic monophosphate. The inhibitory concentrations of TFP were between 10 and 100 microM. One millimolar TFP was toxic to submandibular cells and resulted in decreased levels of cellular ATP and a significant release of lactate dehydrogenase. Determination of CaM levels via radioimmunoassay within various subcellular fractions indicated that the majority of the CaM within the rat submandibular cell was located within the cytoplasm. CaM binding proteins were also identified within these subcellular fractions utilizing a gel overlay procedure. The two major, specific CaM binding proteins present within rat submandibular cells were a 59-kDa cytosolic protein and a 47-kDa membrane-associated protein.