Exchange of potassium and strontium in adult bone

Abstract

The kinetics of exchange of strontium (85Sr) and potassium (42K) were studied in the mid-tibial cortical bone of 37 adult dogs. After injection of these two tracer cations and tracer-labeled albumin into the tibial nutrient artery, two types of observations were made: 1) collection of sequential venous samples to provide the outflow indicator-dilution curves and to calculate the extraction and retention at early times; and 2) detection of energy-selected gamma emissions via a detector over the tibia to give the time course of content of 42K and 85Sr in the tibia. Extractions of K+ and Sr2+ were 50 and 60% during a single transcapillary passage. More Sr2+ than K+ was retained in the first minutes. Their rates of washout over a 3-h period were similar. The interpretation is that the rate of uptake at extravascular sites is faster for Sr2+ than for K+, as is the rate of release, and that the extravascular volume of distribution for Sr2+ (adsorption sites in the interstitium or on bone) is much larger than that for K+ (intracellular).

Therapeutic Radiopharmaceuticals

“…157,167,172 89 Sr is reabsorbed from normal bone into blood and other fluids, as is Ca(II). 156,157,167,172 This is an important property that differentiates it from other bone-seeking radiopharmaceuticals. This redistributive property of Sr(II) makes β-particle microdosimetry with 89 Sr in bone somewhat different than the other "surface seeking" radiotracers.…”

“…The chemical principles provide a foundation for understanding four possible mechanisms for deposition of radiotracers on skeletal tissues which have been described in the literature. The first process involves the alkaline earth metals (e.g., 89 Sr(II)) which absorb to bone by exchange with the surface calcium ions on the hydroxy apatite matrixes (the integral part of the mineral component of osseous tissues). ,, 89 Sr(II), as with Ca(II), is soluble in vivo, and the major process of concern is cation exchange at the surface of hydroxy apatite. ,, …”

“…Ionic 89 Sr(III) has been used to treat painful skeletal metastases for over 50 years 2,7,8 and was approved by the U.S. FDA for routine use in humans in 1995. 89 Sr(II) is an alkaline earth and exhibits in vivo reactivity toward osseous tissues by virtue of its similarity to Ca(II). ,,, While there are differences between Sr(II) and Ca(II), it is generally accepted that Sr(II) binds to the hydroxyapatite matrix by exchange with Ca(II) or interacting with sites normally involved in Ca(II) binding during initial stages of calcification. ,, 89 Sr is reabsorbed from normal bone into blood and other fluids, as is Ca(II). ,,, This is an important property that differentiates it from other bone-seeking radiopharmaceuticals. This redistributive property of Sr(II) makes β-particle microdosimetry with 89 Sr in bone somewhat different than the other “surface seeking” radiotracers. ,,− …”

Nanomedicine Faces Barriers

“…Blood-tissue barriers have been documented and investigated in most tissue types, the following is a brief list excerpted from the voluminous literature. In lung, a less permeable barrier is maintained than in muscle [ 136 , 137 , 138 , 139 , 140 ]; barriers of different “tightness” or “height” are documented in the organs of the immune system [ 141 , 142 , 143 , 144 ], in the gut [ 145 , 146 ], in the hepatobiliary system [ 147 ], in bone [ 148 ] and in the skin [ 149 ], amongst others.…”