Apical and basolateral effects of PTH in OK cells: Transport inhibition, messenger production, effects of pertussis toxin, and interaction with a PTH analog

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Abstract
The cellular distribution (apicalvs. basolateral) of parathyroid hormone (PTH) signal transduction systems in opossum kidney (OK) cells was evaluated by measuring the action of PTH on apically located transport processes (Na/Pi cotransport and Na/H exchange) and on the generation of intracellular messengers (cAMP and IP3). PTH application led to immediate inhibition of Na/H-exchange without a difference in dose/response relationships between apical and basolateral cell-surface hormone addition (halfmaximal inhibition at ≈5×10−10 m). PTH required 2–3 hr for maximal inhibition of Na/Pi cotransport with a half-maximal inhibition occurring at ≈×10−12 m for apical application. PTH addition to either side of the monolayer produced a dose-dependent production of both cAMP and IP3. Half-maximal activation of IP3 was at about 7×10−12 m PTH and displayed no differences between apical and basolateral hormone addition, while cAMP was produced with a half maximal concentration of 7×10−9 m for apical PTH application and 10−9 m for basolateral administration. The PTH analog [nle8.18, tyr34]PTH(3-34), (nlePTH), produced partial inhibition of Na/Pi cotransport (agonism) with no difference between apical and basolateral application. When applied as a PTH antagonist, nlePTH displayed dose-dependent antagonism of PTH inhibition of Na/Pi cotransport on the apical surface, failing to have an effect on the basolateral surface. Independent of addition to the apical or basolateral cell surface, nlePTH had only weak stimulatory effect on production of cAMP, whereas high levels of IP3 could be measured after addition of this PTH analog to either cell surface. Also an antagonistic action of nlePTH on PTH-dependent generation of the internal messengers, cAMP and IP3, was observed; at the apical and basolateral cell surface nlePTH reduced PTH-dependent generation of cAMP, while PTH-dependent generation of IP3 was only reduced by nlePTH at the apical surface. Pertussis toxin (PT) preincubation produced an attenuation of both PTH-dependent inhibition of Na/Pi cotransport and IP3 generation while producing an enhancement of PTH-dependent cAMP generation; these effects displayed no cell surface polarity, suggesting that PTH action through either adenylate cyclase or phospholipase C was transduced through similar sets of G-proteins at each cell surface. It is concluded that apparent receptor activities with high and low affinity for PTH exist on both cell surfaces; those with apparent high affinity seem to be coupled preferentially to phospholipase C and those with apparent low affinity to adenylate cyclase. The differences in apparent affinity of receptor events coupled to adenylate cyclase and the differences in PTH/nlePTH interaction on the two cell surfaces are suggestive of the existence of differences in apparent PTH-receptor activities on the two cell surfaces.

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