Selective macropinocytosis of thyroglobulin in rat thyroid follicles.

We have explored the possibility that pseudopods might provide a mechanism by which newly synthesized, hormone-poor thyroglobulin recently delivered to the follicle lumen escapes immediate reuptake and degradation. The study was performed with electron microscopic autoradiography on rats pretreated with T4 for 2 days and injected with [3H]leucine or Na125I. Pseudopods were induced by the injection of TSH (100 mU) 20 min before perfusion fixation. The density of autoradiographic grains in colloid droplets located in pseudopods was compared with the grain density in different regions of the follicle lumen. In rats injected with radioleucine 1.5 or 3 h before TSH injection, the grains were distributed in a gradient in the lumen periphery. Seventy to 80% of the grains were located over the microvillus region. The grain density over colloid droplets in pseudopods was about 10% of that over the microvillus region and similar to the density at a distance 1-2 microns from the microvillus region. After injection of Na125I, 40 min before TSH, the grains were more widely spread in the lumen, but still formed a gradient in the lumen; about 30% of the grains were associated with the microvillus region. Again, the grain density over colloid droplets in pseudopods was about the same as that at a distance 1-2 microns from the microvillus region. Our observations are compatible with the idea that pseudopods collect thyroglobulin located at some distance from the apical surface. This, together with the circumstance that newly synthesized thyroglobulin is located close to the apical plasma membrane, might provide a mechanism of selective macropinocytosis by which newly synthesized thyroglobulin recently delivered to the follicle lumen is prevented from immediate reuptake.

Beta2-adrenergic receptor polymorphisms at codon 16, cardiovascular phenotypes and essential hypertension in whites and African Americans.

Beta2-adrenergic receptors (beta2-AR) contribute to cardiovascular regulation by influencing several functions and previous studies suggest that a decreased function of the beta2-AR may be involved in essential hypertension. Beta2-AR are polymorphic and certain polymorphisms of these receptors are of functional importance. We focus here on the Arg16-->Gly16 beta2-AR polymorphism, which shows enhanced agonist-promoted downregulation of the receptor and which, in two recent studies, yielded opposite results in terms of association with essential hypertension: an increased frequency of the Gly16 variant in African-Caribbean hypertensives and of the Arg16 variant in offspring of Norwegian white hypertensive parents. In the current study, we genotyped 243 subjects, including both African-American and white individuals, for codon 16 polymorphism and assessed blood pressure and cardiovascular function using impedance cardiography and pressor sensitivity to phenylephrine. We found similar patterns of cardiovascular function and expression of hypertension with the two genotypes of codon 16. There was no statistically significant difference in the overall allelic distribution of the two genotypes: among African-Americans, 51% of the hypertensives and 50% of the normotensives carried the Arg16 allele, whereas among the white subjects 40% of the hypertensives and 47% of the normotensives were carriers of the Arg16 allele. Although we observed a statistically significant increase in the Arg16/Gly16 heterozygotes in the African-American population, the Gly16 allele was not significantly increased in the African-Americans compared to whites. These findings indicate that the codon 16 polymorphisms are not associated with hypertension in a mixed American study population nor do they appear to substantially impact on a variety of hemodynamic variables.

Variability in phenylephrine response and essential hypertension: a search for human alpha(1B)-adrenergic receptor polymorphisms.

Genetic polymorphisms in drug receptors, in particular adrenergic receptors, may contribute to intersubject differences in pharmacologic response. We tested patients and first-degree normotensive and hypertensive relatives of patients with essential hypertension and found substantial intersubject variability in blood pressure response to infusion of the alpha(1)-adrenergic agonist phenylephrine. Because response to phenylephrine depends upon interaction with alpha(1B)-adrenergic receptors, we tested whether polymorphisms in this receptor contribute to the variable responses. Accordingly, we developed a polymerase chain reaction-based method, generating four exon-spanning fragments, to identify polymorphisms in the coding sequence of the two exons of the human alpha(1B)-adrenergic receptor. We sequenced the entire coding sequence of exon 1 from 51 subjects and exon 2 from 16 of these 51 subjects. Compared with the published sequence for the alpha(1B)-adrenergic receptor, we found one amino acid addition in exon 2 at position 368 (Arg) and one substitution (Arg371Gly) in all subjects. We thus suggest we have defined the correct coding sequence of the human alpha(1B) receptor. We found two "silent" polymorphisms in exon 1, one of which occurred in 3 of 51 subjects. These polymorphisms were unrelated to blood pressure status or response to phenylephrine. The 95% confidence intervals for expression of polymorphisms in exons 1 and 2 were 0 to 11%. Our data reveal that although phenylephrine response varies in humans, frequent polymorphisms in the coding sequence of the human alpha(1B)-adrenergic receptor appear not to account for this variation or for the increased blood pressure in patients with essential hypertension.