to ignition agent 190 to initiate the chemical reaction in chemical element 188 the chemical reaction causes piercing element 192 to move to the right piecing barrier 176 as best seen in fig3 b fluid communication is thus established between chamber 172 and chamber 182 through opening 196 which allows fluid 170 to exit chamber 172 as piston 166 is urged to the left by pressure from high pressure region 164 acting on differential piston area 168 communication is now permitted between pressure regions 164 166 via ports 160 162 as best seen in fig3 b referring now to fig4 a 4b a downhole actuator apparatus for controlling fluid communication between pressure regions in the well is depicted in first and second operating positions and is generally designated 250 actuator 250 has an axially extending generally tubular body or housing assembly 252 including two housing members 254 256 that are securably coupled together at a threaded coupling 258 housing member 256 includes ports 260 262 that are respectively in communication with different pressure regions 264 266 slidably and sealingly disposed within housing member 256 is a piston 266 that initially blocks communication between ports 260 262 as best seen in fig4.