[Abstract unavailable

The structural organization of the rostral, lateral and postinfundibular regions of the median eminence (ME) of 5-day cyclic diestrous rats was studied with light and electron microscopic methods. The ependymal cells lining (i) the floor of the infundibular recess (IR) at rostral levels, (ii) the lateral extensions of the IR, and (iii) the floor of the premammillary recess appear to represent the same type of tanycyte ependyma (beta 1 tanycytes). In the entire width of the rostral and postinfundibular palisade regions, as well as in the lateral palisade region of the preinfundibular ME, the processes of the beta 1 tanycytes form a continuous cuff. This cuff separates the nerve endings from the blood vessels and the pars tuberalis. At this level, synaptoid contacts between neurosecretory axons and the ependymal cuff can be observed. The ultrastructural characteristics of the beta 1 tanycytes are described and their ependymal endings tentatively classified into three types. In the lateral regions of the ME, the Golgi study revealed the presence of two fiber systems: (i) one possessing a latero-medial trajectory and distributed in the subependymal region; (ii) the other formed by a loose longitudinal tract originating from neurons of the arcuate nucleus. Some functional implications of the cellular organization of the rat ME are discussed.

Hormone-induced increases in cytosolic Ca2+ (Cai2+) begin as Cai2+ waves in cells isolated from most types of tissue (1, 11), but whether such waves actually occur in vivo is unknown. To investigate this, we examined vasopressin-induced Cai2+ signals in hepatocytes within the perfused rat liver. Using confocal fluorescence video microscopy, we found that increases in Cai2+ began as waves that usually originated in hepatocytes near central venules, then spread opposite to the direction of blood flow, to hepatocytes near portal venules. We used immunochemistry to determine that the liver vasopressin V1a receptor is most concentrated among hepatocytes in the pericentral region, providing the mechanism by which Cai2+ waves originate there. Pericentral-to-periportal Cai2+ waves may direct peristaltic flow of bile, since Cai2+ induces contraction of the apical pole of hepatocytes and since peristaltic contractions in liver also occur in a pericentral-to-periportal direction. The organization of Cai2+ waves among cells in intact tissue may be a means by which an integrative, organ-level response is provided in response to hormonal stimuli.

The kallikrein-kinin system is involved in the inflammatory process, in blood pressure regulation, and in renal homeostasis. The presence of kallikreins, kininogens, and kinins in renal tissues and fluids is well established however, the occurrence and distribution of the bradykinin (B2) receptor in the kidney are unknown. Using chemically cross-linked conjugates of bovine serum albumin and the B2 agonist bradykiuin or the potent B2 antagonist HOE140, followed by antibodies to the respective ligand and the peroxidase-anti-peroxidase system, we were able to detect the B2 receptor, The receptor has been found in straight portions of the proximal tubules, in distal straight tubules, in connecting tubules, and in collecting ducts of rat kidney. The staining patterns produced by the ligand conjugate-antiligand approach are in agreement with those obtained by conventional autoradiography using [1251]-Tyro-bradykinin.

Kinins modulate renal function, yet their role in the developing kidney is largely unknown. To explore the developmental role of the kallikrein-kinin system, we examined the postnatal ontogeny and intrarenal localization of B2 receptors in the rat. Northern blot analysis and RT-PCR documented the expression of B2 receptor mRNA in the kidney and extrarenal tissues of fetal, neonatal and adult animals. The abundance of B2 receptor mRNA is 10- to 30-fold higher in neonatal than adult tissues in the following order: kidney > heart > aorta > lung > brain. Receptor autoradiography revealed a gradual shift in the localization of bradykinin binding sites from the outer cortex in the newborn to the outer medulla in weanling and maturing rats. The almost complete displacement of [125I]tyr(zero)-bradykinin by HOE-140 indicates that the majority of kinin receptors in the developing kidney belong to the B2 type. Immunolocalization studies using antipeptide antibodies directed against various portions of the receptor revealed that B2 receptors are first expressed on the luminal aspect of the upper limb of S-shaped bodies and differentiating cortical collecting ducts. In marked contrast, the metanephric mesenchyme, pretubular aggregates and glomeruli display weak or no B2 receptor immunoreactivity. Following completion of nephrogenesis, B2 receptor expression shifts to both luminal and basolateral aspects of connecting tubules and collecting ducts. The results demonstrate that bradykinin B2 receptor gene expression is activated in the developing kidney and cardiovascular system. The spatially restricted expression of B2 receptors in the differentiating epithelium of the distal nephron, the site of kinin formation, supports the hypothesis that kinins are paracrine modulators of segmental nephron maturation.