Inhibition of transport of 47Ca and 85Sr by lanthanum in canine cortical bone.

Abstract

A critical issue in physiology of bone is whether or not ion concentrations in bone are regulated by a functional membrane that separates extracellular fluid of bone from extracellular fluid of blood. The evidence for the existence of a functional membrane in bone is based in part on in vitro experiments (9, 21) and in part on hypothesis (17, 18, 23). Ramp and Neuman (21) found that both metabolic poisoning of tissue cultures of tibias from Leghorn chick embryos and splitting of the embryonic bone increased the mineral content of the bone. These data were interpreted to indicate that disruption of intact bone led to loss of membrane compartmentalization and allowed accumulation of mineral in bone. Geisler and Neuman’s (9) observation of selective concentration of potassium in bone also suggests membrane activity. The membrane in bone is not well defined. Talmage (23) has considered it to be a continuous layer of cells composed of active and resting osteoblasts and the cells lining the vascular channels of bone, while Neuman and Ramp (17) expressed the opinion that membrane compartmentalization in long bone is effected by periosteum, endosteum, and the walls of the blood vessels in the bone. If there is a functional membrane in bone, it seems likely that a carrier would exist for transport of minerals that concentrate in bone. For example, Lehninger (14) has provided evidence for Ca2+ carrier transport in mitochondria, and others have observed carrier-type kinetics in erythrocytes, squid nerve axon, kidney tubules, and muscle cells, some of which have been reviewed by Bassingthwaighte and Reuter (1). In agreement with Williams’ (26) suggestion, based on theory, that a phosphorylated protein may serve as a carrier, Wasserman et al. (24) have described a calcium-binding protein for calcium transport across the intestinal epithelium, and MacLennan and Wong (15) have identified, in the sarcoplasmic reticulum of rabbit skeletal muscle, a phosphorylated Ca2+ transporting protein (an ATPase) and a nonphosphorylated binding or sequestering protein. Lanthanides may compete for Ca2+ and Sr2+ binding sites on the cell membrane (8) or on a carrier protein (16). Williams (26) pointed out that the similarity in size of this trivalent ion to that of Ca2+ is the main basis for its ability to substitute for Ca2+ at binding sites. Experimentally, Mela’s (16) data on Ca2+ uptake in liver mitochondria suggest that La3+ blocks a phosphorylated carrier. Blockage of a carrier by La3+ implies that it binds to the transport site more strongly than do Sr2+ and Ca2+ and also suggests that the carrier may not transport La3+ across the membrane, because of its larger size. Reuter (22) showed and reviewed evidence that La3+ blocks Ca2+ conductance in cardiac muscle, presumably by attaching to the transport site. This study was performed to determine if La3+ (as LaCl3) will alter deposition of 85Sr and 47Ca in bone. One possibility is that La3+ might cause an increased deposition of 45Ca or 85Sr if it blocked efflux from bone. This probably would not be evident in the early stages of uptake of tracer because the tracer is diluted so greatly in the large calcium pool of bone (3). Inhibition of tracer entry into bone seems more likely, because blockage of facilitating transport sites, whether they be on the capillary membrane or on a specialized bone membrane, would inhibit transport unless there are large, readily permeated, passive diffusion channels that would effectively bypass the cells. A subsidiary aspect was to determine if graded doses of KCN affect deposition of 85Sr and 47Ca ions, an effect that might occur if the transport system were tightly coupled to energy sources dependent on the hydrogen acceptor system.