The county commission of any county in which a special tax for public hospital purposes has heretofore been or shall hereafter be authorized at an election held in the county pursuant to the provisions of any amendment to the constitution shall have the power to designate a hospital corporation in the county as the agency of the county to acquire, construct, equip, operate and maintain public hospital facilities in the county as a whole if the said special tax is a countywide tax or in that portion of the county in which the tax shall have been voted if the said tax is not a countywide tax. Such designation shall be made by a resolution of the county commission, which shall be duly entered upon the minutes of such county commission. A certified copy of such resolution shall be filed with the judge of probate of the county, who shall forthwith receive and record the same. A certified copy of such resolution shall also be forwarded to the State Board of Health. Such designation may contain such terms, conditions and limitations as the county commission may prescribe respecting the hospital facilities that may be acquired, constructed, equipped, operated and maintained by the hospital corporation as such agency of the county and with respect to the maximum principal amount only of securities which may be issued by the hospital corporation; provided, that, anything in such designation to the contrary notwithstanding, such designation shall constitute a contract between the county, the hospital corporation and the holder or holders of any securities issued by the hospital corporation, and such designation shall be irrevocable by the county so long as there shall be outstanding any such securities of the hospital corporation. If any resolution adopted by the county commission pursuant to the provisions of this section fails to specify a particular portion or area of the county in which the hospital corporation shall act as the agency of the county for public hospital purposes, then it will be presumed that the said hospital corporation has been designated in the said resolution to act as such agency in the county as a whole.