Endogenous polyamines modulate Ca2+ channel activity in guinea-pig intestinal smooth muscle

The polyamines spermine and spermidine inhibit L-type Ca2+ channels in whol e-cell recordings from guinea-pig ileum cells (Gomez and Hellstrand, Pfluge rs Arch, 430:501-507, 1995 [4]). To study whether they modulate channel act ivity under physiological conditions, we further investigated their actions on Ca2+ channels and the effects of altered cellular polyamine contents. I n inside-out patches, spermine (0.1-1 mM) inhibited channel activity withou t affecting the amplitude of unitary currents. In cell-attached recordings, addition of spermine to the bath did not influence channel activity in the patch, indicating that its extracellular action is direct and not mediated via passage of the polyamine through the cell membrane. Cellular contents of spermidine and spermine were decreased by about 50% by organ culture of ileum strips for 5 days with the adenosylmethionine decarboxylase inhibitor CGP 48664 (10 mu M). This caused enhanced channel activity in cell-attache d recordings, suggesting a reduced level of channel block by endogenous pol yamines compared with control cells. Whole-cell recordings in the perforate d patch mode showed increased current in polyamine-depleted cells, while th is was not seen when cells were dialysed with the pipette solution. We conc lude that polyamines block Ca2+ channels from the inside as well as the out side of the cell membrane, and that endogenous polyamines in smooth muscle modulate Ca2+ channel activity.