Intercalated Cell

Abstract: The aquaporins are a family of water channels expressed in several water-transporting tissues, including the kidney. We have used a peptide-derived, affinity-purified polyclonal antibody to aquaporin-3 (AQP-3) to investigate its localization and regulation in the kidney. Immunoblotting experiments showed expression in both renal cortex and medulla, with greatest expression in the base of the inner medulla. Subcellular fractionation of membranes, using progressively higher centrifugation speeds, revealed that AQP-3 is present predominantly in the 4,000 and 17,000 g pellets and, in contrast to AQP-2, is virtually absent in the high-speed (200,000 g) pellet that contains small intracellular vesicles. Immunocytochemistry and immunofluorescence studies revealed that labeling is restricted to the cortical, outer medullary, and inner medullary collecting ducts. Within the collecting duct, principal cells were labeled, whereas intercalated cells were unlabeled. Consistent with previous immunofluorescence studies (K. Ishibashi, S. Sasaki, K. Fushimi, S. Uchida, M. Kuwahara, H. Saito, T. Furukawa, K. Nakajima, Y. Yamaguchi, T. Gojobori, and F. Marumo. Proc. Natl. Acad. Sci. USA 91: 6269-6273, 1994; T. Ma, A. Frigeri, H. Hasegawa, and A. S. Verkman. J. Biol. Chem. 269: 21845-21849, 1994), the labeling was confined to the basolateral domain. Immunoelectron microscopy, using the immunogold technique in ultrathin cryosections, demonstrated a predominant labeling of the basolateral plasma membranes. In contrast to previous findings with AQP-2, there was only limited AQP-3 labeling of intracellular vesicles, suggesting that this water channel is not regulated acutely through vesicular trafficking. Immunoblotting studies revealed that thirsting of rats for 48 h approximately doubled the amount of AQP-3 protein in the inner medulla. These studies are consistent with a role for AQP-3 in osmotically driven water absorption across the collecting duct epithelium and suggest that the expression of AQP-3 is regulated on a long-term basis.

Abstract: Microsomal membrane vesicles prepared either from chicken medullary bone or isolated osteoclasts were shown to have ATP-dependent H(+)-transport activity. This activity was N-ethylmaleimide-sensitive but resistant to oligomycin and orthovanadate, suggesting a vacuolar-type ATPase. Furthermore, immunological cross-reactivity of 60- and 70-kD osteoclast membrane antigens with Neurospora crassa vacuolar ATPase was observed when analyzed by immunoblotting. Same antibodies labeled only osteoclasts in chicken and rat bone in immunohistochemistry. Immunoelectronmicroscopy localized these antigens in apical membranes of rat osteoclasts and kidney intercalated cells of inner stripe of outer medulla. Pretreatment of animals with parathyroid hormone enhanced the immunoreaction in the apical membranes of osteoclasts. No immunoreaction was seen in osteoclasts when antibodies against gastric H+,K(+)-ATPase were used. These results suggest that osteoclast resorbs bone by secreting protons through vacuolar H(+)-ATPase.

Abstract: The central amygdaloid nucleus projects to brainstem and hypothalamic nuclei mediating fear responses and receives convergent sensory inputs from the basolateral amygdaloid complex. However, interposed between the basolateral complex and central nucleus is a string of interconnected GABAergic cell clusters, the intercalated cell masses. Here, we analyzed how intercalated neurons influence impulse traffic between the basolateral complex and central nucleus using whole-cell recordings, microstimulation, and local application of glutamate receptor antagonists in brain slices. Our results suggest that intercalated neurons receive glutamatergic inputs from the basolateral complex and generate feedforward inhibition in neurons of the central nucleus. As the position of the recording site was shifted medially, intercalated cells projected to gradually more medial sectors of the central nucleus and were maximally responsive to progressively more medial stimulation sites in the basolateral complex. Thus, there is a lateromedial correspondence between the position of intercalated cells, their projection site in the central nucleus, and the source of their excitatory afferents in the basolateral complex. In addition, basolateral stimulation sites eliciting maximal excitatory responses in intercalated neurons were flanked laterally by sites eliciting prevalently inhibitory responses via the activation of intercalated cells located more laterally. As a result, the feedforward inhibition generated by intercalated neurons and, indirectly, the amplitude of the responses of central neurons could be increased or decreased depending on which combination of amygdala nuclei are activated and in what sequence. Thus, the output of the central nucleus depends not only on the nature and intensity of sensory inputs but also on their timing and origin.

Abstract: The distal tubule, which includes the thick ascending limb (TAL), the macula densa, and the distal convoluted tubule (DCT), and the collecting duct are structurally heterogeneous, thus reflecting the functional heterogeneity that is also present. As the TAL ascends from medulla to cortex, the surface area of the apical plasma membrane increases while that of the basolateral membrane decreases. The structure of the DCT resembles that of the medullary TAL. An excellent correlation exists between structure, Na-K-ATPase activity, and NaCl reabsorptive capacity in the distal tubule. The collecting duct is subdivided into the initial collecting tubule (ICT), and cortical (CCD), outer medullary (OMCD), and inner medullary (IMCD) collecting ducts. Between the distal tubule and the collecting duct is a transition region termed the connecting segment or connecting tubule (CNT). Considerable structural heterogeneity exists along the collecting duct within the two major cell populations, the intercalated cells and the principal cells. In the CNT, the ICT, and the CCD, potassium loading and mineralocorticoids stimulate Na-K-ATPase activity and cause proliferation of the basolateral membrane of CNT cells and principal cells, thus identifying the cells responsible for mineralocorticoid-stimulated potassium secretion in these regions. Finally, at least two morphologically distinct populations of intercalated cells exist, types A and B. In the rat, type A predominates in the CNT and the OMCD and is believed to be responsible for H+ secretion, at least in the OMCD. Type B predominates in the CCD, where it may be involved in bicarbonate secretion.