A medical service plan may provide for medical services to subscribers by physicians and oral surgeons not participating under the plan, subject to the approval of the board of directors of the health service corporation. When a subscriber patient is referred by a participating physician to a nonparticipating physician, the health service corporation is to pay, without the approval of the board of directors of the corporation, to the subscriber, upon proper filing of the claim, an amount equal to ninety percent of the maximum amount which the corporation would be obligated to pay to a participating physician for identical service. If a participating physician refers a patient to a nonparticipating physician, the referring physician shall inform the patient that the physician to whom the referral is being made is nonparticipating and this may result in the patient being subject to charges greater than those to which the patient would be subject if the patient were being treated by a participating provider. The health service corporation shall provide a listing of all participating physicians to all participating physicians annually. The corporation shall provide monthly updates of the listing of the nonparticipating providers.