The Efficiency of Calcitonin Transport across the C-Cell–Blood Barrier in Rat Thyroid

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This paper studies calcitonin (CT) transport across the C-cell–blood (CCB) barrier in normal rats. The efficiency of this transport is expressed as endogenous calcitonin bioavailability (F_CT), i.e., that portion of CT molecules extruded by the thyroid C-cell basal pole that enters the circulating blood. Because currently F_CT cannot be estimated directly, a novel methodology was used for its estimation. The essence of this methodology is to assess the CT level before and after the CCB barrier, to study these input-output CT relationships mathematically, and to find the F_CT range allowed by the relationships. In this context, the subplasmalemmal granule numerical density in the C-cell basal pole (X) and the CT concentration in the right ventricular blood (Y) were found to be highly correlated: r_xy = 0.922, P < 0.001; the covariance coefficient ρ_xy = 0.9980, P < 0.0001. Clearly, the overall variance of these input-output relationships in the covariance model (√(Var_xy) = 6.32) is explained, specifically, by CT retention (R_CT) variability at the CCB transport stages. The greater the R_CT mean value in the studied rats, the greater is the impact of R_CT variability on Var_xy. The estimated variance caused by R_CT (Var_CCBR) makes it possible to determine R_CT and F_CT (F_CT = 100% − R_CT). Thus we estimated Var_CCBR as Var_CCBR < Var_xy − CV_RIA², where CV_RIA² is CT assay variance: √(Var_CCBR) = 6.00% (P ≥ 0.95). We then estimated F_CT as the value range that does not contradict the Var_CCBR obtained: 62% < F_CT < 100% (P ≥ 0.95).