Abstract
Swiss 3T3 cells express receptors for both the polypeptide epidermal growth factor (EGF) and the tetradecapeptide bombesin and respond mitogenically to these substances. These cells thus provide a system to analyze potential signal transduction pathways involved in mitogenic stimulation. Here we have determined and compared the early ionic responses elicited by EGF and bombesin and their relation to diacylglycerol (DG) and inositolphosphate (InsP~n~) production. Whereas EGF fails to cause any significant change in intracellular Ca^2+^ bombesin effectively induces prompt and transient Ca^2+^ mobilization from intracellular stores. Further support of the idea that these receptors utilize distinct signalling pathways comes from the measurements of cytoplasmic pH (pH~i~). As in most target cells, EGF induces a delayed (1 min) but sustained intracellular alkalinization that reaches a new steady state after ∼︁10 min. Bombesin, in contrast, elicits a biphasic response; within seconds, a rapid but transient rise in pH~i~ is observed, followed by a further slower sustained alkalinization. Inhibition of the Na^+^/H^+^ exchanger prevents both EGF as well as bombesin‐induced alkalinization. However, under these conditions, bombesin evokes a rapid and sustained acidification related to the Ca^2+^ response. Apparently, bombesin initiates a Ca^2+^ ‐dependent acidifying process immediately after binding of the hormone to its receptor. Furthermore, we could demonstrate that the bombesin‐induced alkalinization depends on protein kinase C activation whereas the EGF response does not. Determination of the total DG and InsP~n~ accumulation revealed that EGF is ineffective in stimulating phospholipase C‐mediated production of these second messengers. In contrast, bombesin causes a rapid DG and InsP~n~ production coinciding with the Ca^2+^ response and the first phase of the rise in pH~i~ followed by a slower DG accumulation coinciding with the second alkalinization phase. Our results show that in Swiss 3T3 cells the bombesin receptor activates the hydrolysis of inositol lipids as a mechanism of signal transduction, which consequently causes changes in Ca^2+^~i~ and pH~i~. Clearly, the EGF receptor utilizes different pathways to evoke mitogenisis and stimulates Na^+^/H^+^ exchange independently of DG production and protein kinase C activation.