Notwithstanding the provisions of Section 71810 which limit the rate of interest which promissory notes issued by a district may bear, the Otay Municipal Water District may issue promissory notes which bear interest at a rate not exceeding 15 percent per year. All other provisions of Section 71810 shall be applicable to the issuance of such notes. No promissory notes shall be issued by the Otay Municipal Water District pursuant to this section after five years following the effective date of this section, provided that this provision shall not affect the validity of any such notes issued prior to such date.