Not sure if biking once a week will help you (know it won't hurt you); cyclist have great low heartrates because they usually train conscientiously and with plans in mind; ie. they just don't go out and ride 20 miles at an easy pace, they will do intervals, ride long distances several times a week at a maintenance level; do hill repeats, etc., etc. They are training their heart.
It depends on how fit you are now; if you will show any difference by ridinga bike once a week. How you ride, when you ride, how much your ride, what type of workouts will determine how your have trained your heart. Your other activities, i.e. tennis, etc. will also affect your heart rate.