I remember once feeling very sorry for my friends' entire BC when we were camping with them and the family next to us brought in a bitch on heat. He too was normally a well behaved, easy going dog but he had to be tied up from then on and he was clearly going out of his mind being so close to her. He was under control at all times and ignored if he whined, etc but it was excruciating to watch. I am a great believer in the power of training and firm boundaries but I am pretty sure that no amount of disciplining or training would've changed his behaviour much if at all. It was an impulse straight from the penis to the brain and beyond the dog's control. Only physically restraining him avoided him getting into trouble.