apply transparency black pixels mapshow

Hi Everybody.

I have to produce an image with mapshow command, for then exporting on Google Earth.

figure
axesm eqdcylin
mapshow((ax),(ay),(z),'DisplayType','texturemap');
set(gca,'Xlim',[nanmin(ax(:)) nanmax(ax(:))]);
set(gca,'Ylim',[nanmin(ay(:)) nanmax(ay(:))]);
set(gca,'colormap', hot)
export_fig -r800 -a1 figexample.png

The result is here:

How can I apply transparency just to the black pixels and not to the red/yellow/orange ones? Just I need an alpha parameter?

Thank you so much!