Calmodulin antagonists inhibit and phorbol esters enhance transferrin endocytosis and iron uptake by immature erythroid cells

Seven antagonists of the calcium-binding protein calmodulin were found to inhibit iron and transferrin uptake by reticulocytes. This inhibition could be completely accounted for by inhibition of the endocytosis and exocytosis of transferrin. When four of the antagonists were tested with the nucleated erythroid cells from the liver of the fetal rat, inhibition of iron uptake was also observed but at higher concentrations than required for the same degree of inhibition with reticulocytes. The tumor promoters phorbol 12-myristate 13-acetate (PMA) and phorbol 12,13-dibutyrate (PDB) were shown to increase the rates of iron and transferrin uptake by reticulocytes and fetal liver erythroid cells by accelerating the rates of transferrin endocytosis and exocytosis. Since these substances are known to stimulate the calcium-activated enzyme protein kinase C while calmodulin antagonists are inhibitory, it is concluded that this enzyme plays an important role in the endocytosis and intracellular cycling of transferrin, and iron uptake by immature erythroid cells. However, the possibilities that calmodulin is also involved or that the inhibitory effects of the calmodulin antagonists are due to nonspecific actions on the cell membrane cannot be excluded.     

Basilar venous plexus of the posterior fossa: a potential source of error in petrosal sinus sampling

Sampling of serum from the inferior petrosal sinus can provide important information about the source of elevated adrenocorticotropic hormone (ACTH) levels. This often leads to improved results of pituitary surgery for Cushing disease. The authors describe a successful catheterization technique and illustrate the venous anatomy of the inferior petrosal sinuses and basilar plexus.     

Platelet adhesion to collagen types I through VIII under conditions of stasis and flow is mediated by GPIa/IIa (alpha 2 beta 1-integrin)

Platelet adhesion to fibrillar collagens (types I, II, III, and V) and nonfibrillar collagens (types IV, VI, VII, and VIII) was investigated in the presence of physiologic concentrations of divalent cations under conditions of stasis and flow. Under static conditions, platelet adhesion was observed to collagen types I through VII but not to type VIII. Under flow conditions, platelet adhesion to collagen types I, II, III, and IV was almost independent of shear rates above 300/s. Collagen type V was nonadhesive. Platelet adhesion to collagen type VI was shear rate-dependent and optimal at a rate of 300/s. Collagen types VII and VIII showed minor reactivity and supported platelet adhesion only between shear rates 100 to 1,000/s. Monoclonal antibody (MoAb) 176D7, directed against platelet membrane glycoprotein Ia (GPIa; very late antigen [VLA]-alpha 2 subunit), completely inhibited platelet adhesion to all collagens tested, under conditions of both stasis and flow. Platelet adhesion to collagen type III at shear rate 1,600/s was only inhibited for 85%. The concentration of antibody required for complete inhibition of platelet adhesion was dependent on the shear rate and the reactivity of the collagen. An MoAb directed against GPIIa (VLA-beta subunit) partially inhibited platelet adhesion to collagen. These results show that GPIa-IIa is a major and universal platelet receptor for eight unique types of collagen.     

Influence of epididymal maturation on cyclic AMP levels in hamster spermatozoa

Whilst in their native epididymal fluid, sperm from the caput epididymis of the rat and hamster contain significantly (P less than 0.01) greater amounts of cAMP than do sperm from the cauda epididymis. The cAMP levels in both cell types from these species underwent a rapid increase concomitant with dilution to a density of 20 x 10(6)/ml. Further analyses in the hamster indicated that this increase was calcium-dependent, and could be enhanced by treatment with the calmodulin antagonist, calmidazolium. Dilution of hamster sperm to a concentration of 1 x 10(6)/ml was not associated with a rapid rise in cAMP levels. This effect was shown to be due to the dilution of a component in epididymal plasma. When incubated at this lower density, the cAMP content of hamster caput sperm remained low over a 3 h period, whilst similarly treated caudal sperm exhibited a progressive rise in cAMP levels. Thus, in contrast to other species, cAMP does not appear to play a pivotal role in acquisition of the capacity for movement during epididymal maturation in the rat and hamster. However, this nucleotide may be involved in the post-ejaculatory modifications of motility which accompany the terminal stages of capacitation.