Polyamines play a critical role in cell growth, proliferation, and differentiation. The intracellular polyamine pool is highly regulated, and influx and efflux of polyamines are thought to be controlled as part of the cell growth, with increased influx and decreased efflux during periods of rapid cell growth (Wallace & Keir, 1981, 1986). Polyamines also physiologically regulate various plasma membrane channels (Lopatin et al. 1994; Musa & Veenstra, 2003) (Kir, NMDA, connexin (Cx)) by functioning as pore blockers, and thereby play an important role in shaping membrane potential responses and in cell to cell signalling in the case of connexin. In Xenopus oocytes, efflux of polyamines in part follows a simple electrodiffusive mechanism (Sha et al. 1996) through an undefined endogenous pathway. This efflux of polyamines can result in significant accumulation of polyamines in the bathing solution, as shown by the ability of oocyte-conditioned solutions to induce strong inward rectification of inward rectifier K+ channels in excised membrane patches (Lopatin et al. 1994).
In this study, we have examined the possibility that connexin 38 (Cx38) hemi-gap junctions may mediate efflux and uptake of spermidine in Xenopus oocytes. Gap junctions are intercellular membrane channels formed from two hexameric complexes of connexin subunits, known as hemi-channels or connexons, which reside in the plasma membranes of closely opposed cells. Several studies suggest that unpaired hemi-channels are also present in single plasma membranes. These channels are normally closed by the blocking effects of external divalent cations and voltage. However, following removal of external calcium, they open, forming large, nonselective pores that could conceivably allow permeation of polyamines.